To expand, it is something usually used by the newer members to the forums, who think that the first thing to do is go around asking everybody about stairs to find out if they are in fact members. In reality, most members, when met by this question, will respond like any other person: "Yeah, why?" or "No, why?" just to make the person feel silly.
The phrase itself is, in the end, a reference to Dr. Who (I believe), but more directly comes from a chat prank written by the Something Awful webmaster, Lowtax. In it, Lowtax pretends to be a 'pusher' robot who has killed a household of people by pushing them down stairs, and tries to inquire whether it could do the same to the person he is chatting with.
This has been Internet History with TravBot