I'm having the exact same issue.
No @ windows are open at the time.
I'm only using my own scripts, so I know what's in there - nevertheless, I unloaded both files there are, but that problem still persists.
Status window entries in the Treebar collapse or switch places for no obvious reason. It seems to be the same IRC network's status windows that are affected again, even after restarting mIRC.
It's not been like that with mIRC 6.31 - I had just updated for 6.33 when it started acting like that. Maybe that gives someone a clue to what's the actual cause of this.