Moving around (and turning your backside to whatever you're MTing) accidents maybe?
Or maybe after giving your healers a breather with a cooldown use such as Shieldwall?
(I've noticed this one happen to me sometimes; shieldwall, healers adjust, shieldwall expires, healers fail to react; Dead tank )
edit: a good idea after every death (or even after every succesful kill) is to check your combat logs. These often explain a lot. If you're getting a nasty parry streak; that would explain deaths aswell since they'll reset the swing timer on the boss. Stuff like that can all be gathered from your logs.
Hi lol! Turalyon: Asheowyn <Vision> - fury/tank Warrior // Anessa <Vision> - blood/frost tank DK