if nobles make too many mandates in a year, dwarves will riot. During this time, most dwarves will not do any work, and some will essentially be in a tantrum spiral. Dwarves that remain loyal will actively be targeted by rioting dwarves, likely resulting in death. Many assassination attempts will be made on said noble until they fix policy, stand down, and/or die. If a riot is big and loud enough, and an enemy is close enough, they will take advantage of this to try and attack Also some rioters will actively try to light the booze stockpile on fire.
Even if a riot "fails", there will still be a long period of beatings and jail time for everyone involved, the worst offenders getting executed on the spot. Riots can also commonly start over "Guard Brutality", which includes "overly long jail time", "beatings are too severe", and "Executions are overly common". If a Guard Brutality riot gets its way, they will make demands, from "All guards may only equip wooden training weapons and leather armor" to "abolish guards entirely", and if any demand isnt met, the riot will restart with more fervor.
If a "too many demands" riot succeeds in its way, they will make their own demands ranging from "Longer mandated break time" to "This industry is too dangerous/we all find it immoral, so we refuse to do it"
If a caravan comes in as a riot is going on, they will leave rather quickly, possibly dropping their goods as a rogue band of rioters seizes their goods. If this happens to the your nation, they will send in the military to handle the problem... in a much more brutal way than before. You can also send a messenger to incite this response if a riot is too bad to handle.
Other possible causes of riots
General lack of happiness
Religious discrimination (including lacking specific temples, or certain temples being more pronounced than others)
Animal Rights (You butcher too many animals, have too many tame animals, have driven species to near extinction, overfished)
Plant Rights (Your dwarves are useless elf sympathizers who think trees have souls, or you farm too much)
Overworked (dwarves dont get break time practically ever)
Too much death (self explanatory)
Racial Tension (paranoia about a percieved enemy, be it within your own civilization, or an outside threat like goblins, can really put people on edge)