Dangerous behavior. Bucking help?
So I have a 7 y.o. OTTB that I have had for three years and riding for two. We jump, do dressage, and trail ride and he is about as saintly as any horse I've ever ridden. He is calm and wonderful all the time, but sometimes he gets worked up around this one mare, we have trail ridden with her before and he does great just gets fussy after the ride is done and she goes back to her paddock. Annoying but nothing bad.
Yesterday however we went out for a trail ride with her. Before she was tacked up I did flat work with him, and a little jumping over baby X rails to get a little extra energy off of him. He was wonderful over the jumps, and wonderful on the flat. Totally on the ball. However as soon as the mare came out he started pawing and acting like a jerk even though she could care less about him. He calmed down and we went out. About two minutes of walking he throws a massive buck and acts up for a few seconds after the buck. I stayed on made him walk and he calmed down again. About two minutes after that I was walking him in large lazy circles while the mare and her rider did smaller circles a few yards away. I brought him to a standstill for two seconds before asking him to continue at a walk when all the sudden another bronc fit came out of no where and he got me off. He then proceeded to run around bucking and rearing and going nuts almost running into the other rider. I finally got him to stop and caught him and walked him calmly back to the barn (He's 17.2 hh and I can't mount him from the ground, I'm very short) after we got to the barn I took him to the round pen and did liberty work for about 10-15 minutes. I took him into the barn, remounted and proceeded to do more flat work and jumping again. AND HE WAS A SAINT!
The point is, he gets dumb around this mare, but this time he was flat out dangerous and I cannot find an explanation for it. I wasn't forcing him to be away from her, I wasn't struggling with him. We have gone on rides with her tons of times before and he's never done this. He's not a hot horse most of the time, he gets a small amount of grain and I work him before we go out of the trail. This might sound like a small incident but this is extremely odd behavior for my horse. I'm just wondering if anyone can give me a little insight into possible causes. He's a gelding but I'm beginning to wonder if he still has some testosterone bouncing around in there. I don't know his past so I have no idea if he was gelded before or after he raced.
If anyone has any tips, advice, stories or anything else it'd be really appreciated.