There was a previous criticality incident with the “Demon core” on August 21, 1945 Harry Dahlian was messing with the core and induced an energy excursion killing himself and injuring others. After the Slotin excursion the core was melted down and used in other weapons.
Not confirmed but good pilots think it was spatial disorientation during the change from takoff configuration to flight configuration. The pilot(s) took their eye off the ball and got bit. They were in the deep soup.