the wow engineers confirmed that sometimes the server doesn't agree with your client in how your character moved and it will result in a disconnect.
- Move enforcement code runs on the server to make sure you aren't moving in a way that should not be possible. The current code gets this right about 99.9% of the time. The rest of the time some issue causes the server to think you moved in a way that isn't possible and you will be disconnected. Druids had a patch of water in the Order Hall that would cause a disconnect if you were on certain mounts. These are sometimes very difficult to track down.
however their 1 in 1000 odds seems completely unrepresentative of how often it actually happens.