I've had this happen to me twice now, only in BG's, where I've taken enough damage to kill me but instead of killing me it leaves me a state of limbo as it were.. Where I can move around but NOTHING else, I can be attacked by players but it will not kill me as I am already dead as it were.
It won't let me do anything, I've tried relogging with this bug, alt f4, everything aside from leaving the BG altogether. Although when the BG does end it does fix the bug, but it doesn't help me when inside the BG I literally cannot do anything while this bug is in effect.
No idea how it happens so there really isn't much help I can give as to the cause but as funny as it watching horde try to run around killing me when I can't die it would be alot more enjoyable if I could participate in the BG.