Apollo has been playing really rough with these 2 female Labs. They get along great, have known each other for 3 years, lots of play bowing and body bumping and tug of war, and in general rough play (not really mouthy though)- but all 3 play like that. Lately he's been nipping them, usually in the butt, and I don't know why. Example, if one of them starts doing zoomies he will run after them, they will freeze, play bow, maybe bark at each other, run around and if he catches up to them he nips them, usually in the butt! He only does it to these 2 females. When he does nip them they sometimes "yip", then they stop their game, ignore him (even though he keeps trying to play), but after about a minute they will go back to playing again. It's so weird because he just started doing this more often....any ideas? I don't want him to hurt them, he doesn't really seem to be. Also, the entire time he's with them his tail is high as a flag- I mean straight up. Normally it isn't.
I don't want to end his playing with them because they all seem to enjoy each other. Never any fights or aggression- ever.