A poster on another board noted a very high volume of shares trading after hours on Friday, that's typically an indication of being added, or removed from an Index where certain Institutions must hold shares in that index.
In the past, I've seen the MM's fill such orders, and the shares short go up practically by the entire number of shares added by the institutions. I have no qualms against what they do initially, but I've also seen them hold the short position for years, eliminating it when the company was removed from a Russell Index. I do object to maintaining naked short positions for years. For all intents and purposes, the MM's are creating stock which no one but the MM's are paid for.
SGEN has had Institutional ownership in excess of 100% for years, essentially the MM's are holding a multi million naked share position in SGEN in permitting this to happen. No telling how many Institutional owners are also short the stock.
Gary