Here is the chart:
The carry trade (where people borrow at low interest rates in Yen and invest in dollar-denominated assets) is only profitable when the dollar is strengthening against the Yen.
If the dollar weakens against the Yen, the risk is that not only are your profits wiped out, but you might not be able to buy back enough Yen to repay your loan.
Trump posted at 4:50 EST on Friday 11th October. The foreign exchange markets were still open: they close at 5 p.m. New York (which is 10 p.m. London).
Those ten minutes were enough to weaken the dollar. Stock markets were closed, but crypto markets were open. Any crypto automated trading bot that was financed by borrowing in Yen, would have automatically have started closing longs as the dollar weakening made those longs unprofitable.
It cascaded from there, as stop-losses were triggered.
Unfortunately too many crypto traders use leverage. And too many are tempted by borrowing in low-interest Yen because interest rates in the US are so high.