User blog comment:Omega natsu2/Episode 203: Moulin Rouge, Review./@comment-4449950-20141011035137/@comment-108.28.233.191-20141011054643

Them using the flame pants is likely a function of censorship while still trying to get in some fanservice. The omake chapter just had Bisca steal Erza's panties, which I suspect is something that didn't fly for the timeslot. So they tried to keep the joke by having most of Erza's outfit stolen and her trying to keep her modesty with what was left, then switched her to something else for the rest.

They likely opted for the flame pants because it's the simplest to draw.