Shorting doesn't only mean short stock. You can be short call options, and if they are in the money, the holder of the contract can exercise his or her option, which forces the seller of the contract to provide shares at whatever price.
For every long call option contract, there is a counterparty who is subject to unlimited upside price risk, just as a traditional stock shorter. The difference is the contract has an end date.
I would argue that options account for more of the pressure/volatility of these recent stock prices than the shares themselves.
