This was the episode that made me want to write Starbuck in the style of Bridget Jones fic. As a joke, of course, don't shoot.

Beyond that, I have no answer to the mystery of the origin of the dress.

I think she wore it to get Lee to notice and then, oops, he noticed, and he noticed a lot, in a rather slack-jawed manner, and she spooked. And since she is all manner of fucked-up, and self-destructive, she went for something that was easy and attachment free. It also feeds into her self-loathing, when Lee gets so insanely furious about it later. Reinforces her "I'm a fuck-up, see how much? stay away from me, I'm not worth it" thinking.

jmo, of course.