Mom went crazy from stress, caused by shitty spouse. (understandable, things happen) locked Amy in the car, for reasons unknown. Maybe she actually thought she was going to get ice cream or smth, maybe she didn't know what she was thinking at all. The whole thing seems weird since she seemed to be very attached to and dependent on Amy, so it's possible she didn't any of this consciously (knowingly? Whatever conveys that she was not in her right mind, that's what I meant) at all.