Is that one account spamming more things than the others? The only thing that comes to mind is if you pass some threshold of bandwidth or something like that, they will boot you. So if you're in the heat of battle and spamming DPS, and this guy is spamming a lot more than the others, he could be hitting that.

Otherwise no, definitely haven't heard of anything like that. You can try clearing out your WoW cache files (noting that to avoid file locking issues, ISBoxer keeps different cache files per window, in the same location as the originals)