I seriously doubt it cooked off. And I've had weapons cook off in my control.
If it cooked off there would not have been a firing pin dent on the primer.
And while the grip safeties failed all the time, the trigger had to be engaged (pulled) in order for the hammer to get past the “half cock” position.
So you placed the gun in the holster, thumb safe off and something in the holster engaged the trigger when you touched the side.
Watch this video at the 3:20 mark it animates a 1911.
https://www.youtube.com/watch?v=E6SmlOEzNBs
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