In Mt Hyjal it often sends you to a spirit healer on a differnet level from where you died (i.e., over the edge of the cliff way down the bottom). Since it can't (easily?) get back to your corpse, it just spirit rezzes you. That said, I've also had it spirit rez me when I'm on the same level within viewing distance of where I died on level ground, so its definately a bit buggy in that area.