This chart shows Bitfinex USD margin shorts and on the bottom the changes in these positions, denominated in BTC.
These are enormous positions which clearly have an impact on the market. The recent 25k BTC short position was solely responsible for pushing price down about 6%. That position must have had a value of about 1 billion USD.
So Bitfinex shorts might be something bitcoin traders should pay attention to.
Extra tip: a lot of people don't know this, but there are also Bitfinex UST shorts which use tether as collateral, not USD.