At some point Annie has had enough of looking at Sunstreak's messy wings. She and Danny differ on a lot of things, but something they share boldly is that they like to keep things tidy, especially when it comes to personal hygiene. When she has a chance, she takes it upon herself to get rid of those horrifically mangled feathers that clutter up Sunstreak's wings and make his scars itch. While at first he insisted she didn't have to, he couldn't complain once the uncomfortable feeling of bent and broken feathers went away.
Plus, it's hard to argue with a mare that has very sharp teeth within biting range….