User blog comment:Miskos3/Fairy Tail Episode 240: Tartaros arc: Where Prayers Go, Review/@comment-26287497-20150704140222/@comment-7418318-20150704162223

Or we could just apply a bit of logic.

They are in enemy territory, so being prepared is a good thing. If she already is in her take-over, she doesn't have to waste a few precious seconds to transform and instead can attack immediately.