This is really concerning - if the trees are private, and you haven't given anyone (editing or contributing) access via the Tree Sharing option, it shouldn't be able to happen.
Can I just ask if the trees have been previously shared, just to rule that out as a possibility?
And I assume you will be contacting Ancestry to ask how this could possibly have happened - will you keep us posted on their reply?
Sympathies to you - this is one of the things we all dread.