In many respects, riding on MUTs is more dangerous than riding on roads. You certainly can't let down your guard just because you're on a trail. I've had similar experiences to what happened to you, although I avoided collisions. I don't hesitate to holler if someone is riding unsafely on a MUT, and I try to carry a bell. Glad to know that you weren't seriously hurt. Whoever was in charge of that tour should have taken charge, although ultimately the rider who doubled up was to blame.