there are still a couple of Market Makers (MMs) in PTSC. MMs are required to "make a market" in a stock, to provide liquidity and efficency in the trading of that stock. That responsibility will at times mean that they build inventory of a particular stock, regardless of price, by buying it when there is an absence of (mainly retail) buyers, or to sell a stock when no sellers offer their shares for sale. Sometimes they sell stock they don't have in inventory to order to keep an active and orderly market, and that means at some point in the future they have to go and buy it back from the open market to square their books.
Read up on Market Makers, this is basic stuff.