It's not surprising why this happened. BitMEX offers 100x leverage for their Daily contracts, and there was a fast $50 drop in its price, which meant that people who were long and did not have margin to cover their position got liquidated:
To those who don't know, socialised loss is what occurs when you're trading counterparty futures and an individual gets liquidated and the order which results from the margin call is not filled, leaving a loss in the system.
This loss is spread among the profitable traders in order to reach a balance, meaning anyone who profited on the XBT24H daily contracts lost 24% of their profit.
This is the standard way that OKCoin, the current #1 volume futures exchange, has dealt with unfilled liquidations from high leverage futures trading, and BitMEX adopted it when shifting from a hedging product focused model to a more high leverage speculation model.
Despite offering 100x leverage, BitMEX has an impressive track record of no socialised losses. Their settlement page shows 0% for all adjustments going back to October, so this was the first time it happened, which shocked a few traders.
Immediately the troll box exploded with butthurt traders who were either ignorant of the rules or simply stubborn about the consequences:
Socialised losses are just a fact of life in high leverage counterparty trading in bitcoin. No exchange wants to take the risk of these unfilled liquidations driving system losses, and it's reasonable that those who profited well in the contract period have to pay to keep the system sustainable. It hurts, there's no doubt about that, 24% is a bitch, but what about the 2 months of DPE-free settlements before that where traders were going 100x and enjoying the integrity of it? Their mark price system has made socialised losses really rare and allowed for really smooth trading at extraordinary leverage.
There's really no way around having a backstop like this. Socialised losses is a Chinese innovation that has stood the test of time (at least in bitcoin terms, nearly 1.5 years is pretty good). BitMEX tried to go against it and offer lower leverage but they eventually went to the "dark side" and now they have many traders and are doing very well volume-wise, landing in the top 10 at Exchange War.
So don't look at this post as a defense of BitMEX, but as a defense of the socialised loss system. And please, when you've made great profits on a contract, don't cry about taking a small hit for the system's sake.