Not all of what goes on in a thread is visible to the general membership. Sometimes a closure results from a number of deletions or edits when it becomes apparent that trying to keep it on topic will be (or is already) a losing battle.
Some topics are predictable death spirals and there's no need to watch until final impact and detonation before taking action.
Sometimes it's clear that a member (or more than one member) is heading for trouble and a thread will be locked to let things cool down and prevent the need for more drastic action.
That said, if you see a thread that you feel shouldn't have been closed, or that you feel should be reopened, you can feel free to PM the staff member who closed it and ask (preferably politely) about the possibilty of reactivating the thread. If you can make a coherent argument as to why the thread should be reopened, there's a reasonable chance of a positive result.