Suddenly spooky horse

I bought my gelding 1 year ago. 5 yr old QH. I rode him both in the arena and on trails. We on trails I was 98% alone and he was rock solid and willing. I also have been taking regular lessons to grow our communication and partnership.

In May, he tripped at the lope, and we both went down. We both weathered it well, with me getting banged up but nothing broken. In July, we were in the arena at a walk. He swung his head to bite a fly on his side, bumping into my foot in the stirrup, which bumped into his side making him jump sideways and I came off. I thought reaction to that was a little large for the situation.

While I healed from that, I decided to have 30 days put on him to find any holes in his training etc. My trainer did plenty of gro6nd work including buoys on the saddle for more exposure to things on his sides if that was the issue. He has done beautifully.

I rode him Monday and had a great lesson. He was light, responsive and relaxed. I was at the walk at the end of the lesson, again relaxed, when he spooked seemingly out of nowhere with no cause we can see. I had surgery yesterday to put my collarbone back together.

I scheduled the vet to come out Tuesday to evaluate him. Something doesnt seem to add up to go from solid to jumping out of his skin.

We do have a ton of ticks a d a few horses have been tested and treated for Lyme.

He is also cinchy when being saddled since I have owned him.

Any thoughts on other possible causes of the reactive spooks?