The bank of Japan raised their interest rates above 0 for the first time in like forever.

This has caused the yen to strengthen vs other currencies.

Because Japan had rates below 0 for so long, traders would get paid to borrow yen and then they’d trade those yen for other currencies and buy equities or other assets, to yield profits, and then they would trade those assets for profits and exchange the currencies to yen to the pay off the debt they borrowed from Japan

Since the yen is strengthening, and it’s no longer free to borrow yen, the forex exchange and borrowing costs ate into the profits hurt the trade, causing some to get margin called forcing them to liquidate their assets. This forced liquidation then cascaded lowering the value of equities overall, combined with a large amount of people in the trade, a run for the exits ensued to prevent greater losses