There was no particular reason for it - we knew the strat; we'd downed him a couple of times, but we still continually wiped on him to the point where people /gquit over it, thus bringing raids to a halt for lack of numbers (BC had been announced, so the upcoming expansion and holidays also contributed).
In BC, Leo was a problem.
We may have killed him and moved on to the "harder" raids, but I still don't feel we killed him enough times with ease for the fight to be considered "comfortable".
Right now, my guild's arch nemesis is Thaddius (or perhaps the frogger gauntlet after Patchwerk, if you're counting non-bosses). Someone misses the ledge, someone dies on the first transition, someone gets DCed... and we end up wiping at a few measly % HP due to enrage.
There seems to be no particular reason why; it's not the same people dying each time. Everyone knows the right way to run around and positioning is fairly good (although I've had my share of rounds where I've muttered under my breath about melee not standing far back enough so that hunters can get the full complement of stacks), and again, we've killed him a few times. It just seems like this fight is more difficult for us than any others.
I suppose guilds with The Immortal title would find this a foreign concept, but it'd be interesting to see if Thaddius in particular causes problems for most other guilds as well (due to having to rely on others more than usual), or if everyone else's guild has a different arch nemesis (Heigan perhaps?).
In any case, I'm thoroughly sick of Thaddius, even though I used to love killing him (40man Thaddius and Heigan were two of my favourite retro fights). The day when my guild can joke around while doing it can't come soon enough.