Working Theory
Karen Read and John O'Keefe get into a very heated drunken argument so John gets out of the SUV. Karen has a road rage moment and throws it in reverse to go after John, maybe to scream at him, maybe to hit him. She clips him or she misses him. John goes into the house, everyone's wasted, he tells everyone what happened with the SUV. They either joke about it or he is critically injured. They can't call an ambulance because there were lines on the mirror. A fight later breaks out, Shepherd Chloe defends and bites John's arm, John is killed, or he succumbs to his injuries from the SUV. Everyone agrees, just blame the SUV, whatever it takes, especially given the scenario where he was already injured by the SUV. They bring John outside, get rid of evidence, and they are too wasted to talk to police at 2 AM. They figure just claim to find him in the morning.
Chloe the family German Shepherd, the family dog of 7 years, was given the boot several months later.