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!).
Once phase two hit, she flew up and then immediately started to breathe fire onto the raid. This behavior in particular really was quite odd, since the fire breath isn't supposed to happen right away, and you're supposed to get a warring before she does it – so you can run to the sides and away from her. Nothin' doin'. She just started this automatically. Really quite odd, and frankly annoying.
Now Ony is a "boss" level mob, so that means that she's going to attempt to crush me. I never really noticed if she did, since Shield Block is part of my ingrained rotation. But beyond that, all of her attacks are susposed to be level 60-ish, which they were. That combined with my 21.5k buffed health meant it was all pretty easy on the healers.
One time when we attempted her she decided to walk around the welps cave (Welp! More Welps! Right Side!). She stood there looking at us while we dot'ed her up and ranged her down until phase 2, but again, same result as above – insta death from the flame breath.
We ended up never getting her (50 dkp minus). Archimode? Not an issue for us. But Ony? Watch out! Any other stories of pre-bc boss fights gone insane?
And since I've been alluding to it my whole article, here is the NSFW Onyxia wipe animation.