Very high percentage float...Many shorts are now either low equity or unsecured.
They were not on friday...that is calculated by mortgage brokers the next morning before market opens and margin calls are issued and due in a few days in some cases.
In other cases where the account is low equity, the positions in the account are either sold at open with or without contacting the customer, regardless of their intentions of covering the amount due. Perfectly legal to secure their loan. Read your margin loan agreement or call your broker and they will explain it to you.
Some brokers will watch the position and close the position at the close of the day if no wire comes in. It is at their discretion and the short does not get to decide.
If it is a regular margin call, they have a couple days to buy back, but they can not short any more as long as there is an equity situation.
In other words, the short sellers are now the best buyers in the market. Since many of them thought it would dip, this caught many of them off guard. Add the puts expiring next friday and the market makers will want to keep all the premium from the 15 puts.