We had a horse who used to be silly like that. Not always at the same thing and often because he was on his own and that was his way of saying, he was out of his comfort zone. A very annoying a somewhat dangerous way, I must admit.
We found out along the way, that some of it was programmed into him. We spoke to the previous owner one day, and we had motorbikes go along the road behind us and he took no notice. She was amazed, because, she said, he always freaked and bolted for home with motorbikes. The problem was, she had decided he would be afraid of certain things and so he was. When I was on him, I didn't act any differently no matter what was going on and if he did play up, he got the 'get over it and get on with things' message.
He did get a lot better, but he was never going to be a 'solo' horse. He needed to be with company .
Only As Old As You Feel - Sometimes I Feel VERY Old