What would make a sheriff want to follow someone to find a reason to pull them over?

Obviously, I can't know the answer to your question. I would be as irate, if not more so than you. Being in law enforcement, I can only guess. Perhaps you, or our car, look like someone/thing that they have been advised to be on the lookout for. It still doesn't excuse them. The next time it happens, why don't you ask, in a polite, nonthreatening way.