Pheets may be on to something. Have you had a vet look into his eyes? You can do it yourself. You know that little tongue-shaped thing (corpora nigra) you see when you look into the iris? Well, in some horses it can get big enough to actually be visible to him and cause him distress as it waggles within his field of vision.
As Pheets says, however, that would only explain his spooking, not the bucking he’s associating with it, which is, I contend, a learned behavior either because it’s fun or because he might have had a pain issue and is simply recalling that. Remembered pain is my big Paint gelding’s stock in trade, and it’s been a long haul getting him past it.
Horses In the Yard