My 10-y/o gelding 2-Pak has always been a pretty friendly with other horses... He's never come across a horse he doesn't like (although it seems plenty of horses dislike him). But since we've gotten Tanner and Magic, he's developed an odd behavior... if either one of them is in heat, he absolutely despises her. He'll run her off if she gets close to him, or Bandit and Dante, and he won't let her eat or be petted.
He's never bothered Ruby -- or any other mares -- while she's in heat, and if Tanner and Magic aren't in heat, he's back to being best buddies with them.
He's never gone so far as to actually injure either one of them, just lays back his ears and chases them away, or nips at them a bit. Neither of the mares bother him, in or out of heat, and they're both older than him. They're usually just standing around and grazing when he decides they're an insult to his prescence or something.
I can't say I've ever heard of a gelding doing this, and I was just curious what might cause his behavior.