New topic`s are changing places on their own ??

When the members in my forum posted new topics this weekend, their topic moved itselves to other treads , and not where the member originally placed it...

I do not really have a clue about running a forum , so at first i had a little panickattack . I have moved the treads back to the right category where they belong, but i can still se this beeing a problem if this happens in way to many treads.
Have been sitting her for a few hours moving them back manually myself..

Somebody who knowes why this happends, and what i might do to prevent this for happening again ?

Anybody who can help ?