Based on my experience with forums, likely no one reported it.
Mods do not read every thread, they do not need to. But when a thread goes from a few posts to almost 1000 posts in just a few days, a mod will notice. Usually a thread reaching 1000 posts will take weeks or months.
As a mod, when a thread gets that big that fast, it means the topic of discussion is highly charged and needs to be looked at. It is highly unlikely that someone reported the thread, though it was such a sensitive topic, it may very well be true.
As for thread moderation and deletions, it is all laid out in the guidelines.
"Thread deletions
We don't generally delete whole discussion threads, unless they're libellous or racist or otherwise break the law, because it's frustrating for people to write carefully considered responses that, minutes later, are removed. That said, we're here to make people's lives easier, not the reverse so if a thread derails or becomes seriously unpleasant or a bunfight, we may decide it is best to delete it.
Also, if you have a pressing reason for wanting a thread you have started to be deleted, please report it to us (by clicking on the Report link, above and to the right of the post in question) and we'll take a look.
If a thread is deleted, please resist the temptation to start a new one, repeating and rehashing everything that has been deleted: that would be very bad manners and, of course, it's highly likely that the new thread will be deleted, too. Similarly, we will delete threads about other ongoing threads, as it causes too much confusion on the forums"
I suspect that this thread will also be deleted because it about a deleted thread.
