It's probably because she is bugged. And not just bugged, REALLY bugged.
Two groups have now gone in and tried to defeat her. Each time the first attempt was a failure as people relearned not to stand too close to each other (Forsythe run to the center!), run to the side when the rains fire down (<-- 2N, 2N - 1 -->), and all those other small tidbits that make the fight what it is. However when it reached the second attempt, she started acting strange. First, she wouldn't target anyone or let anyone tank her – she just walked around her lair and jerked around a bit. We could still damage her, but that was it. And really it was only the ranged and my crossbow that were able to do any damage (more dots!).