we can ask and explore those. might get some good theories.

heres what i think

when dollars or stables get scarce on a platform the spreads between market price and platform price widens.

in this case the price was way lower than market which means there was almost no available usdt on bitmex.

available could be an "absolute" thing -> there was almost no usdt on the platform

or available could be a "relative" thing -> whoever had the usdt didnt want bitcoin

in bitmex case it was probably an absolute thing, as their such a small player these days based on spot volume (66m last 24 hours in coingecko). while most of the OTC desks and asian market makers have an account there, they would have loved to arb this right? of course, but who wants to keep collateral on such a small exchange? no one.

if someone knew this, they could spark a run with spot sells, step out and then be the liqudiity backstop on the other side.