I suspect there is no real reason for them doing it beyond the fact that they can. They might think they're helping the market, or helping newbies, or it's just for giggles. Maybe this is how they craft in bulk or something? (You can reach a point where 20k vs. 2k is a meaningless difference, so why not just bid at 20?) Maybe it's none of these things. ¯\_(ツ)_/¯
Regardless, it's happening, and it's been happening for weeks. Also, it's not the first time it's happened (though it's perhaps the most sustained effort I've seen). Regardless, I'd encourage folks to take advantage of it, because it could stop at any time. I notice some of my own under 20k bids filled recently, so, at some point since I last posted, the floor was lowered, but, as I type this, it's pinned back at 20,002 again.