I wonder if there is a possibility the gun fell into the back seat from a holster or something. Not really an excuse, since she still lost control of it.
Police determined the gun had been on the rear floorboard. She was driving when shot. Her son retrieved it from the floorboard at some point, meaning he wasn't secured in his child seat when he did it. So either he was unsecured while the vehicle was moving, the gun was accessible to him on the floorboard when he got into the truck, or the vehicle stopped along the way and he got to the gun while unsupervised. Catastrophic fail regardless. The gun didn't just fall into reach of his hands.