When you sell a security you are generally selling it to a MM and they have to technically short the stock if they don't have any on hand.. or have a client selling at similar level. These MMs need to buy back the shares borrowed in effect making them naked shorts. This all stems from an increase in PPS and volume making the shares scarce to begin with (MYDX yesterday) and forcing MMS to start selling shares they dodn't have to keep market liquid. They still get to pick their bids and asks but it pays to keep shares moving so they will "short" or sell you shares they don't have today, and buy back later or within their margin call threshold. They can easily flip the market back another day to cover the shares they "sold" to their clients buying. Buying them the next day when there is less volume, makes it easier to move the price to a favorable buying PPS. So there are technically shorts, just not the way you or I would short, or be able to do for that matter. It does backfire when the market rallies for a few days though so they do have some risk, but they keep our market liquid and we love liquid don't we.
Wow sorry that was long but people need to know market mechanics better. Hope you learned something today folks. Thanks for reading.
