The battle is going poorly for the would-be thieves, so one of them focuses all of her energy into a devastating counterattack.
Artist note: This is pretty much how all our battles go. Our GM is very clever about designing his NPCs with some devastating nuke power, so if we don't win the battle in a few short turns we get smacked down hard. It does make the fights suspenseful for us, because we know it's a countdown to doom.