Personally, I think she accidently hit him while drunk and very angry. I think she had no idea until she saw him lying there in the snow, then she realized what probably happened.
One possible scenario is they were fighting on the way over, both nearly blacked out drunk. She scratched him leaving marks on his arm through his thin sweatshirt. Growing up with 3 sisters I can personally attest this is possible. He gets out of the car, and does something to set her off, like kicking the back of the car, breaking the tail light, and loosing his shoe. She starts to drive off but the rage kicks in and she does the backup at 24mph. He dives out of the way, or is glanced by the car, transferring pieces of the cracked tail light onto him while he falls backward and hits head on the curb. The end. And you can imagine multiple other reasonable scenarios, none of which the mythbusters that testified this week tested.