I can imagine that if she panicked, she would have done something less than ideal.
One time I was on my way to pick up a friend from work. It was like... 10pm. I stopped in a Kmart parking lot that was relatively empty, so that I could feed dd (she was maybe... 4 months old at the time). I was sitting in the driver's seat feeding her, and the car was still running and everything (I didn't expect to be very long anyway). This van starts driving around my car and STARING at me.... and then yelling at me. If he had come back around one more time, I DEFINITELY would have just taken off -- baby in my lap and everything. I was surprised, panicked, and it probably really was a dangerous situation. Thankfully, the man drove away before anything more happened.
SO..... being in a similar situation, I can say that in the heat of the moment, I would have just taken off without much thought, you know? I imagine Britney may have panicked at the idea of how much time it would take her to get her son back in his seat, and that they would be surrounded by that time, etc. Even with someone else in the car, I can understand it happening.
I think that I probably eventually would have handed the baby over to the person in the car though, while driving.... after being able to think for a minute.