I suspect that the mother dropped the child only after being struck by an open door or otherwise pulled off balance by the vehicle suddenly moving. The greatest negligence, IMO, was in having the thing in neutral in the first place.
That's one plausible explanation. The article linked further down the thread references being struck by an open door.
I think the point I and others were trying to make is that if you leave an SUV in neutral, clamber down from the driver's seat to the ground and then open up a rear door and lean inside to unsnap a child from a car seat the car is likely going to noticeably have started into motion by the time these various actions are underway.