You don't need to put up signs saying you have cameras up, you need to put the cameras up.
And you also need to consider the possibility of someone running your horses, or tormenting too them, not just possibly "riding" them.
Much has been in the news lately about horses being shot with guns, arrows, and last year some in TX were run to death. Some have been stolen, and not that long ago, some moron put a young horse in an office at the barn, and tormented it to the point that it went berserk and tore out the wall of the office, severely damaging the horse too.
Your horses may be getting beaten across the back which would account for some of what you are seeing with the bucking.
You need to make a report to the sheriff's office, and also the houses that border this land need to be checked out by you or parents. I'd also go look to see if someone is coming through woods?
Might also post around to see if others in this area, county are having these problems, as chances are very good they are. I'd also contact any vets in area to see if other horse owners have reported these problems, or have any suspicions about someone bothering their horses.
And check with your farrier too and ask him about whether anyone else has had this happen?
Horses make me a better person.