In this video at 0:54 you can hear him singing just before the accident. Doesn't look like a medical issue. Highly likely that autopilot was on (because the car was perfectly centered in the lane all the time) and he wasn't watching the road. He had maybe 4 or 5 seconds to react.
Wow, so the local media has the dash cam footage of the crash, but the family won't let Tesla have access to the vehicle?