MM or Market Maker is the "guy" who makes each stocks market and handles the bid and ask.
When our stock runs upwards, our Market Maker will have trouble filling all the buy orders the demand. So he can sell shares into the market to fill orders. The rule is that MM cannot keep a naked short position any longer, they have to buy back the "phantom" shares they sold into the market to fill all the buy orders or the demand for the stock.
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