It's most likely an inside job, either by disgruntled employees or SBF himself:
"In a subsequent examination, FTX legal and finance teams also learned that Bankman-Fried implemented what the two people described as a "backdoor" in FTX's book-keeping system, which was built using bespoke software.
They said the "backdoor" allowed Bankman-Fried to execute commands that could alter the company's financial records without alerting other people, including external auditors. This set-up meant that the movement of the $10 billion in funds to Alameda did not trigger internal compliance or accounting red flags at FTX, they said." [1]
I kept wondering how someone who seemed smart, wanted to help others, and admitted that crypto is a Ponzi scheme could end up in crypto. It broke my brain.
Now I see he was a brazen criminal all along and the world makes sense again.