The way that market makers work is they keep the price constant between the spot and derivative exchanges if the funding/futures is near flat. So to prevent a huge arbitrage event the market makers who got accounts on the index spot exchanges like Kraken, Gemini, Bitstamp and Coinbase they buy/sell to keep the index the same as the mark price.

So if there is a huge dump on fake leverage, they will follow and that's why its dangerous. Most of the selling on Bitmex was due to long liquidations and it hit a low of $3596. And if you look at the spot exchanges like Kraken, Gemini, Bitstamp and Coinbase they hit a low of $3913, $3901, $3850, and $3858, respectively. So obviously the spot exchanges were following Bitmex.