The random movement, from spamming IWT, is warcraft related.
Nothing you can really do to prevent this.

Well - not spamming the key is one solution, which reduces how often this occurs.
But in PvP, you pretty much have to spam the key.

Once a toon is running off, you can end this with any movement key, such as down arrow.
You can also initiate follow, if your toon is fairly close to the leader.
Or of course, have the toons interact with a new target (IWT range is 3x further then follow, so they'll probably be in range of the new target).