Sometimes (very rarely actually) when you join an instance, you randomly see players stuck on the spawn, not moving at all but able to hear them talk while in fact they aren't even standing at the spawn.
I ran into this issue twice, but haven't taken a look at the logs yet. Friends of mine also had this issue, most of them just once or twice, too, so this isn't a local problem. Once this happens again for me, I can check the logs and see if anything related pops up.
Workaround for this is to just rejoin the instance, the problem is so rare that it's almost guaranteed that the next time you join the instance, you're fine.