You have to look right at the seat it seems for it to happen, looking at the side of it you sit normal.
When you get up and zoom out the camera is on the back of your head but you are facing forwards, i could not flip it back around without respawning. It seems to be triggered either by just sitting in that position or zooming out while in that position.
[Note this pictures was taken later in another server, when i realized that is what caused this bug.]
I am walking forwards and when i zoom it it is forwards, but as you can see my model is facing backwards. I am not moving the camera in any way with my mouse.