We've been reporting this bug since Phase2. Every single time it gets filed in "insufficient information".

Most people assume it happens when positional data for the "invisible" player stops updating properly for you (the other client). The server obviously still knows where the player is, they're still fighting, the mobs know where they are, etc. Only the other clients lose the data about where that player is. The positional data "resets" and updates properly if you log out and back in again, or if you die.

You were lucky you were dps when it happened. Try healing when you can't see the tank...