Hmm I thought I would contribute.
I've had some horses become very "hot" on particular feeds and then be fine on others. Like my horse went absolutely insane on SafeChoice and it wasn't until we swapped to a similar feed that he was fine with the same amount.
It could be an allergic reaction, it could be high protein or too much digestible energy (as in they're getting lots of hay and pasture, so since they aren't working hard as let's say a seasoned 7 year old, they can bounce off the walls if given more)
I don't know for sure, but just to let you know it happens!
"Strength is the ability to use a muscle without tension"