The thing is, it can, in many cases, be put up, or removed in one click. So unless they have impeccable timing, they are usually one player. At least the majority of said wall.
I'm pretty sure the big walls are mostly built by bots. I wouldn't want to be responsible for manually typing and clicking to put up or pull down a huge order. I would rely on a bot to do it in milliseconds.Heh, I don't know if I'd trust a bot to do it.
If the algo is just slightly off, that's one potentially expensive mistake.