An educated guess is the guy got one of the belts of his restraint system tangled in the seat initiation handle and tried to pull it loose.
Tragic. One would think there would be a switch housing that’s cover had to be raised manually to access the switch. I believe that’s standard in Navy A/C as per NATOPS. I suppose if he was wrestling with his restraint system and exerted upward pressure to open the box and caught the toggle just right he could defeat that safeguard.
I’m basing the above on knowledge I picked up from sitting on a court martial board. The same is true of fuel dump switches.