Whenever I am around other riders I don't know, I will err on the side of caution... I assume they are novice riders and act accordingly. That is the safe thing to do.
On the other side of the coin, there are "experienced" riders who like to baby their horse and except everyone to tiptoe around them because they either have a perceived problem with their horse, or refuse to address a genuine training problem. I am NOT claiming the OP falls into this category. Please don't misinterpret this as any type of attack on anyone.
My point is that this is a two way street. Riders should not be running up on unknown horses. It is not safe. At the same time, riders (especially trail riders) have the obligation to train for the unexpected and learn to deal with it.
You could be riding down a trail and a runaway/loose horse(s) could come riding up on you. A deer, moose or bear could come out of a side trail. A tree could be blocking a normally clear trail. There are an infinitely number of variables and situations that are completely out of our control.
People should observe "trail etiquette" but the reality is that sometimes they do not. Best bet is to observe it ourselves and prepare for/expect that that no one else will.