Of course we’ll never know - what a big bummer.
In general when diagnosing things, the human operator is most likely cause, then electrical, then hydraulic (doesn’t apply), and finally mechanical.
Dialing the wrong number under pressure is pretty common. Hearing/seeing one number and thinking another. Do you ever transpose numbers in your head? Just getting the correct body/rifle angle must have been a challenge, which is a flashing warning.
Wonky reading with the laser wouldn’t be unheard of. That’s a steeper angle than almost any of us take readings at which stands out as a flashing warning - I’ve had electronic construction level finders give a wonky reading, but a second or third reading was correct.
Mechanically, other than losing zero, a 3+ MOA first shot shift is a lot - I’d think it would take a baffle strike, a big chunk of flaked off carbon in the muzzle, a week firing pin barely igniting the primer, or an ammo issue. Wet freezing conditions could have a little ice in the bolt slowing down the firing pin - the time it happened to me the firing pin had to fall three times to set it off and the bullet landed almost 2 moa off.
I’d chuck it up to one of the mysteries of life if you can’t find an obvious answer.
Still, you’ll remember that one 20 years from now.
