When a dwarf has been unhappy enough, he'll decide that enough is enough and he'll throw a tantrum. Tantruming dwarves will cancel any job they may be doing at the moment and will start misbehaving, which includes throwing items around, starting fist fights (with the possibility of severely injuring other dwarves), toppling and destroying buildings or hurting pets. This is of course illegal, and they will be imprisoned or flogged for their behaviour. But even though they are punished, if their punishment is more harsh than usual, their death (or the results of their misbehaviour) can cause another wave of tantrums, and your dwarves will begin a process known as a
downward spiral of dumbness tantrum spiral which will almost certainly result in fun.
Tantruming may be a warning that the certain dwarf may go insane unless something makes the dwarf happy.
To make tantrums less likely, see the article on keeping your dwarves happy.
Tantrums only occur when a dwarf is "Miserable", the lowest possible state of happiness. In this state, a dwarf has a 1/1000 chance each frame (about 70% chance per in-game day) of responding to its unhappiness. A dwarf's vulnerability personality trait determines how likely they are to respond - a lower value makes the dwarf less likely, with a value less than 25 making the dwarf effectively immune to unhappiness.
Each time a dwarf responds to being unhappy, its "unhappiness" counter is incremented; this value becomes a percentage chance for the dwarf to go insane each time it responds to unhappiness (unless it has successfully completed a strange mood, in which case nothing happens). If it does not go insane, then its anger personality trait is checked to determine how likely the dwarf is to actually throw a tantrum - a lower value makes the dwarf less likely to tantrum, with a value less than 25 allowing the dwarf to maintain its composure until it goes insane.
Once a dwarf becomes happy again (at least to be no longer Miserable), it has a 1/1000 chance each frame of decrementing its unhappiness counter, making it less likely to go insane the next time it tantrums.
(This requires lots of friends, married couples, and the oh so common
bad luck fun.)
- Dwarf A, a craftsdwarf, gets the urge to build a mysterious construction, but unfortunately there are no bones or shells to be had.
- A goes berserk, kills dwarf B, and wounds dwarf C. A is then struck down by dwarf H, making an unhappy thought for A's friend dwarf E.
- B's loved one dwarf D, and friends E and F, get a strong unhappy thought.
- D, going to help C, throws a tantrum. He kills C in his rage, giving another unhappy thought to C's friend F.
- F, now horribly unhappy from the death of two of his friends, throws a tantrum destroying E and D's beds.
- E, now very unhappy from two of her friends dying as well as losing her bed, goes melancholy and commits suicide by jumping down the well, giving an unhappy thought for D and J; and contaminating the only water source.
- D, despite the happy thought for fighting (and killing) C, is overwhelmed by unhappy thoughts and tantrums again. This time he destroys a bridge, drowning F in the moat (as it has no ramps), and then attacks dwarf G.
- F's friends, I and J, both get an unhappy thought.
- E's body, inaccessible, rots, causing J to tantrum. He punches G, hospitalizing him.
- D once again punches someone, this time I - cutting his lip.
- Without fresh water, G dies a slow death of dehydration.
- I, even more angry due to being punched, punches D back.
- D, finally being overwhelmed, goes stark raving mad.
- I, angry about being D's punching bag, punches J, and J punches H.
- I, Unable to wash his lip with clean water combined with all of the rotting corpses, succumbs to infection.
- D dives into magma, creating a unhappy thought for D's friend, H.
- H, a skilled warrior, in the wake of death and Miasma finally goes berserk and finishes the fortress off.
Fortress at the start: 10 "normal" dwarves.
Fortress after tantrum spiral: 9 dead dwarves, 1 berserk dwarf (likely soon to die from starvation/dehydration). Causing fortress-stopping Fun.
As a result of not having the right bones or any shells, your fortress falls. The main thing that started this mess was Dwarf B's death. And bad luck. Isn't losing Fun?
 Damage Control
It is possible to survive a tantrum spiral, but it's going to take some serious damage control.
- Prioritize making alcohol, coffins, and food over everything else. Stop any unnecessary work.
- Bury all that die, burial lessens the unhappiness of death, and prevents miasma and ghosts.
- Handle the most unhappy dwarves with special care. Giving them additional rooms and furniture could slow down their happiness decrease. Relieve them from work and let them meet their friends, pets and relatives if they have any. Expedition leader/Mayor/Baron etc. could sometimes give them consolations, or be yelled at, so make him available for meetings. If situation around a certain dwarf seems unresolvable, try to isolate him while you can, preferably with walls.
- Move military inside to kill anyone that berserks. You'll die if an ambush hits you during the spiral anyway.
- Create Jails by placing rope or chains in small rooms. Under Build (b) Restraint (v), then select the restraint using (q) and select Use for Justice (j), much like you would when placing a bed and making a bedroom. This will place people misbehaving or throwing tantrums in jail. Then if they do go berserk, they will be restrained away from everyone else till they calm down. When a fortress starts Tantrum spiraling make as many Jails as needed.
- Also, if, by a stroke of luck, some already-happy migrants come, then they will add to population and you will now have dwarfs who will do work without being interrupted by the need to destroy everything.
- Confining a dwarf to a burrow will prevent him from starting a fist fight. generating instead the announcement 'Urist McAngry cancels Starting Fist Fight: Target inaccessible.' This only works if his chosen target is outside the burrow designation and is extremely useful in preventing a tantrum spiral, as it prevents any death or injury resulting from the fight. This is presumably a bug.
Keeping your dwarves happy is an important task. While it's easy enough to keep everyone happy under normal circumstances, trying times, like the aftermath of bloody goblin ambushes, will push any fortress dangerously close to a spiral.
Having a high overall happiness helps, but the best on-the-spot prevention is solitary confinement. Lavish room and board arrangements into which the dwarves can be locked will prevent their aggravation from spreading, as their upturning of chairs will not affect any of the other fortress occupants, as well as working wonders for even the most hardened of prima donnas.
 Prevention, second method
There is another alternative way to avoid tantrum spirals. It is quite a bit more difficult, but generally far more dwarven and !!FUN!!.
The whole point is to expose all of your population to enough violence so that they take the trait "doesn't care about anything anymore".
Most of your military eventually acquires the trait naturally. This trait makes the dwarf not care about death, so they will stay "quite content" and will not generally not throw tantrums.
You have many choices for exposing your dwarves to violence. Dropping dogs, cats or poultry 15 z-levels on top your meeting zone seems to be the safest way to do it. You can also make your whole fort in a military / as hunters and make them kill animals or enemies, but this is quite long and unwieldy.
Since a dwarf with this trait does not tantrum, tantrums and therefore tantrum spirals will become exceedingly rare, and if you manage to immunize your whole fort, not only tantrum spirals will not happen, but general dwarf mood will be equalized, leading them to not care (much) about Spartan living conditions.