He gambled. Lost. Gambled more. Lost bad "even when times were good". My guess is he figured if he could borrow from Peter to pay Paul one more time... one more time that he would eventually be back in the black. I think he personally set up those "positions" over and over, but he's not good at gambling so now it's all gone.
https://www.nytimes.com/live/2022/11/30/business/sam-bankman...