This was my thought exactly. If this was your first trail ride EVER on him, you threw a lot at him all at once (even if this horse has been ridden out before, I would treat him as if he were green and build up his confidence in you). The horse sounded like he was doing fine at first and then became overwhelmed. If the other horses with you also spooked, it's quite probably that they heard or smelled something, but by the time that happened, your horse was probably already mentally exhausted by all the new things that you had asked him to do.
When I start taking horse out to trail ride, I ask a rider with a steady Eddy type of horse to lead and we walk. We walk on a loose rein if possible and horsie gets lots of praise for being brave. If this is oh-so-boring then we trot. If horsie is good, we come home and he gets lots of praise. I stick to wooded trails (no open fields) and we try to end on a high note. I want the trail ride to seem like fun. Gradually I introduce more challenging obstacles -- crossing water, trotting or cantering in open fields, riding in a group, etc. I try to make sure that we add only one new thing each time and break it down into something not so scary. Some horses need me to hand walk them on the trails or past scary objects.
It sounds like you treated your horse as if he was a confirmed trail horse. Trotting, cantering and jumping (even an 18" log) is a lot to ask of a horse and it is likely that those activities got him worked up. Being in a situation where you ask horsie to do something different from the other horses (not jump) also can trigger anxiety. I generally ask the person I go out with on my initial rides if they mind being a babysitter and scaling the ride to the needs of my horse.
It sounds like what happened to you was truly scary! It does not mean that your horse cannot event but it does probably mean you need to go back to basics and teach your horse that you are the alpha and that he needs to listen and trust that you will keep him safe. Next time you go out, I suggest going with one horse, keeping it short (15 minutes) and making sure that he has a good experience even if that means only walking.
I have had 4 OTTBs (and have a new one right now) that had never been on trails before. Each of them turned out to be fine on trails although a couple of them were quite nervous/overwhelmed at first. Sometimes they seemed fine initially but lost it halfway through a ride. That usually means I asked too much of them. The one I had now went on his very first trail ride yesterday and was a star. Sure, he ground his teeth and jigged a bit but for the most part he followed his friend even remembered to breathe occasionally. The slow and steady approach generally takes the least time in the end and is the least scary for all!
Good luck!

