A similar situation to this happened last summer at a very popular trail ride event - we're talking literally more than 100 horses and riders, raffle, lunch, etc. The trail opens and closes at a certain time and people are welcome to start whenever they want to, so everyone's fairly spread out.
What if you were riding at a trail event (not necessarily competition - just an "organized trail ride" and you come upon a parent and child each riding their horse. The child's horse is acting up, including kicking and biting at other riders' horses. The child is obviously very upset and uncomfortable with the situation and cannot handle the horse, yet the parent keeps trying to "coach" the child for what they should do. All the while, they are coming up behind other riders as well as having other riders come up behind them, constantly making the situation worse and putting other people and their horses at risk. They are creating a sort of road block with this stop-act up-go thing, and when you try to go around, the horse acts up and kicks out, but when you're ahead and want to slow down, they catch up and you're still in the mess (not to mention the issues going on with the parent and child as well). Unfortunately, since it's a trail ride, there are no "officiators" to step in and do something - so what do you do?
In my case, I just did my best to get out ahead of them and my horse has a big trot so I put quite a bit of distance between myself and this situation. I didn't necessarily want to be trotting the whole ride, but I couldn't slow down even if I got a decent distance ahead of them because they would catch up again. Of course, this horse didn't have a ribbon in its tail. Is there ever a point to step in? Does it make a difference that there are other riders in danger? Or do you just watch out for yourself and hope everyone else does the same?? And pray that kid somehow doesn't get thrown in the process?