ingame reasoning is that she probably vaguely remembered that when it's hot outside it's fine to go around in "lighter clothing" as long as your privates are covered, but probably realized at some point that most humanoids like to cover their chests too, just to be polite.
Real talk, I just hate what the bra looks like on a dragonborn.. they don't have anything there! what's there to support and/or hide?? 🙈