The title says it all. This isn’t a bug, but a design flaw, though I believe it has its place here.
If a horde or remaining ambients are alive and everybody goes to the arena and the cinematic triggers, said enemies will attack the helpless players during the cinematic.
If I dare make suggestions in how to fix it, the cinematic could forcefully stop enemy aggro, force players to parry (still not enough in the case of CWs or Maulers, though), close the arena, give invincibility to players, only trigger after aggro’d enemies are cleared, be removed, et cetera.
Possibly. Will re-test. I ran into the issue on recruit and therefore didn’t pay attention to my health, and for Cata we always cleared everything before stepping in. That said, this doesn’t prevent you from being dragged by a packmaster, as I’ve seen.