Why would they lie? Obviously they could of, but quite frankly, I see a little bit of that "equestrian entitlement" attitude showing.
I could definitely make my horse bump into someone - I couldn't make her "trample" someone, but she would obey me if I made her bump into someone. This COULD also be innocent - a poorly trained horse got spooked, and in bumping the rider and another horse being reined in and tossing his head, this could have been a lousy misunderstanding.
At the end of the day, you don't know, but I find it difficult to believe these bikers just hate horses so much they made it up. I KNOW of people ignorant enough on a horse to do something like this. And not saying sorry is pretty ridiculous, regardless of what happened.
When I lived in CA and had horses we used to go up to that trail as well as another one "whitings Ranch" and in CA there is a HUGE issue with mountain bikers vs horses. Most MB groups complain about the horses having right of way over them because they feel they should have the right of way because they go faster and have a harder time stopping at such speeds. The horse people say they should have right of way because a horse is more unpredictable than a bike. It's been an argument in CA for over a decade. So we have two groups of people at war with each other.
In this scenario I see a possibility of the horsepeople being a little rude to begin with and the bikers blowing their initial rudeness out of proportion. This is what tends to happen when waring societies are invited to the same party...the Capulets and the Montegues, The Jets and the Sharks, the Hatfields and the McCoys. You have them both in the same place, something is GOING to happen and somebody is going to exaggerate it to his pals to escalate the whole war and to make themselves feel like they have the right to feel the way the do about the others...
I think it was a misunderstanding of sorts, and probably got a little out of hand because of the war between the two.
On a note, I do think horses should have the right of way. They have a brain to think about and be scared of said bikes, while bikes just stop when the rider hits the brakes. That being said, ALL trail-users who take such a trail should at least take a trail ettiquette class as well as learn how to control/handle the horse during a situation like this. The horse should never have gotten close enough to the bikes in the first place.