Our county runs finals days at Maidstone FC. Pay one entry fee and potentially get to see three finals played one after the other, all at a decent ground (3g) with decent facilities. The last Sunday finals day a team from Thanet played a team from Medway and there was a mass brawl behind one of the goals just after half time. This was actually the showpiece final, so the two top Sunday teams in the county's couldn't control themselves. Luckily there were only a couple of hundred at the game. The County FA were mortified, as was my local league where one of the teams involved plays.
I refereed the game a few years ago - a local derby - and there were about 600 present. SE Dons played the final before last and brought 2,000.
It's a potential problem because non-league sides generally don't need a police presence until at least National League level, but the crowds at some of these games are getting as big as those seen at games where police attendance is mandatory. As you say, it's a matter of time before there's a major incident.