You give this explanation every time, but every single time it's because no, the thread is actually mysteriously locked. This is like the third time it's happened!Which threads?
If it's because of the lock icon next to some thread titles that's because the lock icon is used for "hot thread with new posts" as there isn't another icon for that. Or something. Menty explained it once. It's confusing.
Also the "close this thread" check box is next to the "show your signature" check box for staff members so sometimes it gets accidentally clicked.