If your vehicle suddenly stalls (engine stops) on an incline, as his did, the power steering would become inactive and it would be difficult to steer the rolling car. Power brakes also become inactive. It probably drifted over the center line as he pulled the emergency brake. Unable to start it, it would be left there.
There's nothing erratic or unusual in his behavior. The autopsy/ medical exam didn't show the presence of alcohol or drugs. He was probably walking back to the police car explaining what happened, but they weren't listening. The guys in the helicopter, on the radio, had already pegged him as a dangerous person. He was probably confused by their reaction towards him, but raised his hands and started walking back to his car. They decided to tase him instead.
There were many ways they could have de-escalated the situation, but they chose not to.
I can't figure out why the woman officer shoots him then yells "shots fired" into her radio. Perhaps she was trying to cover up and make it seem like he shot at her first.
These officers have already been caught giving false information about this incident. Right now, I don't believe anything they say.