I figured he was used to crowds and just looking for something out of the ordinary that might have spooked him. No telling. Horses funny. Mine acts up about the tractor even though it brings him hay all the time but today rabbits kept running across the arena into a field we were riding. I was ready but it never phased him even though they would even surprise me, when I knew they were in the grass.
On the other hand, we were riding in the arena when two of the barn kids came down the hill about 100 feet away giving each other rides in a wheelbarrow. She saw that apparition and reared up and plunged forward - luckily I was in position and had my reins fairly short or I would have just rolled off over her tail. The other thing that frightened her was when some guys near the arena were loading 50 gal drums and dropped one off the tailgate of a pickup truck. It was partially full of liquid, and as it rolled down the slope it wobbled and made sloshing noises. The mare stood stock still and stared at it, but she was trembling so hard I got off as I was afraid she would lose her footing and fall down! I patted her and turned her around so she couldn't SEE that horrible thing, and after a few minutes she calmed down. But she gave the big beady eye to the black oil drums we use for jumps in the ring for a LONG time afterwards, expecting them to start rolling and sloshing . . .