"Why it happened cannot be determined..."

Agreed. But he was also in charge of the Wing's Standardization and Evaluation branch, which means that somebody up the chain-of-command was favorably impressed with his flying, and had charged him with impressing upon those in his organization his distorted sense of values and standards. That opens up a whole new chain of responsibility for the accident.

I think the OP was asking about the "why" from an aerodynamic perspective. How much altitude does it take to recover a B-52 from a 90 degree bank at minimum airspeed? How much from a stall in that condition? I doubt many B-52 pilots know, but I would guess at least 10,000' would be as good a place as any to start.