I think some of the changes in Pie's behavior, including his biting one of the other boarders, are arising out the transition he's made from still a baby horse to a mature horse. Since I got him last November, he's grown several inches and has filled out and put on weight - he no longer looks like a young horse. His herd status has changed, too. When I got him, he was bottom of the herd in rank and often initiated play with the other geldings - he acted like a baby. Now, although he ran from Drift chasing him when Drift was in with the others, Pie's the dominant gelding - I saw him today moving Scout around and he's also now dominant over Fritz, the old alpha, who's been disabled by injuries and foot problems and is in his 20s. Pie also now rarely plays, but is more serious in his demeanor.
I think that it's likely that, now he's discovered he can move up in the horse world, he's trying on some things in his interactions with people - some of this may also come from the fact that he was handled extensively by people other than me - who may not have been as strict about boundaries as I am - while I was laid up. He knows where he stands with me and the p.m. barn lady, and has never offered to bite either one of us, and continues to lead very nicely for me, including on a long walk we took together yesterday. There was that odd episode when he pinned his ears at me, but I've been careful to always "answer" when he "asks" by pinning, nudging me or moving into my space - when he pinned at me in the pasture that time I immediately moved him back. I expect what happened with the other boarder is that he "asked" her a question and she missed the ask and perhaps was in a hurry or not paying attention, and when she didn't give him an answer, he thought he'd try to take things a step further and do what he'd do to another horse - a firm bite to move her out of his space. Since she didn't manage to immediately react, he's learned he can move her, and he might well try it on again with her or with someone else - we'll be keeping an eye out for that. I've tested his demeanor and behavior on a number of occasions since then, by approaching him while he's grazing loose, for example, and in various grooming/leading/paddock/stall situations, and he's been just fine for me.
His increased "lookiness" when ridden may also be a sign that he's made the transition from baby (following the leader without much worry) to an adult horse who has to be in charge of himself and aware of his surroundings, and who may need to think about which human leaders he's willing to follow. That's a natural stage for a horse to go through and I think we'll work through it just fine.
Pie is a personable horse, and easy for me to work with, although he isn't what I'd call cuddly - he's a bit standoffish and self-contained. I think his basic somewhat "cool" personality, combined with his new-found adulthood, goes a way towards explaining his behaviors, both with horses and people.