We don't know that.
I have a dewalt jig saw at work, the trigger lock button is placed in such a way that I pretty near each time I use it lock the trigger on. I know its there, but the ergonomics of the handle dictate how I hold the thing, and most times I end up locking it on. Extremely annoying. Back around 1990, had a makita hammer drill, trigger lock was a small knob on the trigger, this drill was well used (daily use hammer drilling) and the trigger lock, depending on which hand you happened to be holding it with would lock on as well with ease. Stupid design with the trigger lock on the trigger.
In this guys case, when the drill got away from him, did he inadvertently activate the trigger lock during an attempt to keep control of the tool? Quite possible.
Alex.