If the other person on the line was not the neighbor and didn't call 911, he probably ended the call after noticing someone in the driveway. If a person I didn't know was standing in my driveway, I'd probably confront them before opening the garage. If the window was up when he was shot, did he not even confront this person? Pulling into the garage gives this stranger access to the locked house, but maybe he thought the remote garage door would close faster than it could. If he didn't think the guy was a threat, it makes more sense. I just can't imagine taking the risk that the garage door would shut without this threatening person sliding under.
I do wonder if the neighbor saw more than LE is sharing with the public. If this person was just waiting in the driveway, I bet the Prius was parked on the road. It seems like a sloppy contract killing but maybe its not. The hit man could have been dressed as a UPS guy standing in the driveway with a package, which wouldn't be immediately threatening, or out of place.
ETA: If the neighbor discovered him in the garage, the garage door was open. So, the hit man probably appeared to be nonthreatening. Mail carrier, landscaper...someone who wouldn't seem out of place in the driveway.