In this case, they are the homeboys. This is their territory, and they want to be sure she understands that she's an interloper. Every new horse starts at the bottom of the totem pole, and has to work their way up. Your more subordinate pony is feeling a bit smug about being able to dish it for once . . . .
In my experience, it takes about 2 weeks for the 'new' to wear off. When I worked at a boarding stable, I got used to the idea that new horses are crazy; they are all going to try things to see what they can get away with. Once they had it in their heads that I was in charge, I managed to get along fine with almost all of them. I saw this out in the pasture, too - the new horses got grief from almost everyone, unless they proved too "bad" to be beaten down. There was one gelding that I particularly remember; a bulldog-type Quarter Horse named Moose. The first time he was turned out in the geldings pasture, he was the first one out. I turned a few other geldings out, they all had to go over, sniff noses, squeal, etc; though there was some horseplay, nobody actually made contact. Then I turned the mares out in the adjacent pasture. One of them was in season, and she went over to the fence near where Moose was. She was peeing, and flirting, and doing the things that mares do. Curly, the most dominant gelding in the pasture, couldn't stand it - he was one of those "studly" geldings, and as far as he was concerned, all the mares on the place were his, never mind that they weren't in the pasture with him. He came flying across the pasture and landed right in the middle of Moose. There was some squealing, a few kicks and bites, and within about 30 seconds, Curly retreated, bleeding in about a dozen places. I caught up with him in the place he'd gone to regroup, and checked to make sure there weren't any serious injuries. There weren't, and I asked him, "well, did you learn anything?"
