Burt Likko, a lawyer who did work for an insurance company “which wrote liability policies to a whole bunch of seedy bars,” relays what he learned about the topic:
You might think that a bar fight is most commonly started between two guys fighting over a woman. That’s not so, at least not in my experience. Ejection seems to be a more precipitating event. More than half the bar fights I had to sort out started when a too-drunk patron was asked to leave and refused to do so. When the bar back or the bouncer attempts to escort the drunk out of the building, the drunk refuses to cooperate, and if the escorting turns in to physical handling, the drunk will wrestle away and attempt to run back in the bar. It is during this struggle that harmful physical contact between the drunk and someone else is initiated. By whom is not always clear — does the drunk punch the bouncer; does the drunk flail at the bouncer and hit a bystander; does the bouncer hit the drunk? These are the burning questions that must be sorted out in a bar-fight lawsuit.
He also shares his thoughts on the link between bar violence and sexual frustration:
[B]ased on what I heard from dozens of witnesses, those bar fighters who initiate confrontations with other patrons (as opposed to reacting badly to being 86′ed by the staff) do so as a substitute for obtaining sexual release. Dozens of witnesses over multiple cases reported to me that the person they identified as the assailant had either recently suffered a romantic reversal or had recently stuck out when trying to hit on a member of the opposite sex.
This suggests at least some substance to the trope of a link between propensity to violence and sexual frustration — the woman (or man) with whom the drunk was flirting is already spoken for and uninterested in extra-monogamous play, typically. “I’m not very likely to get laid today, so instead I’ll fight with someone,” seems to be roughly the thought pattern here. I speculate that this means on a neuro-biological level that engaging in aggression and violent behavior produces a feeling of satisfaction, which in turn triggers a release of endorphins or other similar hormones that the fighter’s brain craves.