A horse can't verbally tell us when they are hurting or something is wrong. So they act out. They will kick, buck, bite - do things not normally in their character in order to say, hey - I don't feel okay.
So whenever a horse's behavior changes that first thing I do is think medical related. Then I go backwards to environment (feed, other horse's, other people) and work out what it can be.
And honestly the majority of the time, it's been a medical related thing.