It depends on what they were there for to begin with. You can route your buying and selling through specific market makers. One of the ones that showed up, OTCX, is gone today but I think they are a new market maker. PUMA is still on board though.

My best guess is someone wanted to build a position, they routed it through a specific MM, they showed up, accomplished the mission, and now they're gone.

Keep in mind too, China is buying now.