If M&M's are net short a position after a recent run, they need to cover because those short positions were created to fill a market that was lacking sellers on the way up. To fill all the long share requests, the M&M's naked shorted shares to fill longs orders.
As volume decreases when the run starts to peak or slow, M&M's can buy and sell between them lowering the bid and ask as they do so. Then panic selling among longs can contribute to the shake speeding the decline as M&M's buy those shares to cover their naked short positions.
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