Member group changing

I have some strange behaviour occuring.

I originally had one private forum and all members were in a special group for that forum. However, yesterday I created a new private forum with new groups for that forum. All seemed OK except that one member of the original forum group kept getting his group changed to the new group so couldn't access his forum. There are only a couple of very trusted admins set up and they all confirm that they have not changed the user.

I changed the member back to his corect group, but a couple of hours later he was back in the new group again.

The application sits within a corporate intranet, so it is highly unlikely that it is being hacked externally, and I have changed the default admin password.

The db is MS SQL

Thanks. Easy when you know how