The way respawning is done is because of a very stupid "in combat" system they use in Flashpoints. If any player in your party gets aggro from anything in the flashpoint, the rest of your group, no matter where they are, will be put in combat and those mobs the player aggro'd will go to them after he's killed/uses a vanish ability. So even if you are respawned at the entrance to the flashpoint the boss will walk all the way over there because the game still says that you are engaging him.