The fact that he curled his finger on the trigger and squeezed it as he drew was the reason the gun went off.
Ah, but WHY did he curl his finger? Because he was still putting pressure on in the same area and fashion he was tripping the release switch.
Look, if you tell your finger to curl, and put on some pressure, telling it to STOP isn't the quickest thing in the world. Right? So that's a really terrible place to have to apply some pressure.