What you’re seeing there is simply lag between the wow clients and server. You need to be aware that your clients are constantly updating the server with movement information; the server will then send instructions to each of your clients telling them to update the movement of each character on screen. In order to make this appear smooth the client is allowed to continue to draw the movement of a character even when the server is not instructing it to if it received a prior movement command and has not been instructed to stop; you may notice this sometimes when one of your characters appear to run past your group and then snap back into place. In your case the client you were watching had been instructed to move the characters on screen in a certain direction, but then as it does it is updated with a delayed command that tells it to turn the characters, however that turn occurred back some distance so it snaps them back and does the turn. This behavior continues to happen until the information starts to arrive in a timely manner again.

This may well be out of your hands, it could be caused by any number of issues between your system and the Blizzard servers. If you only experience this sometimes it is likely that your connection is capable of handling it, but you may want to take note of the time of day that it occurs; if you’re in an apartment complex it could be that more people are online and saturating the limited bandwidth. It could also be something on your PC starting up and using the network connection. There are a lot of reasons this could happen but most are out of your control.

This is ultimately a very watered down explanation, there's a lot more involved and understanding it fully requires a deep understanding of internet protocols and how the game client and server interact.

- Aldercy