I think in the initial scene he's just a young kid taking the car for a joy ride...I suspect that leads to some bad behavior on the part of his Uncle. He responds by taking the car out and deliberately driving if off the cliff.
I've pretty well given up thinking we'll get Kodos or Tarsas or anything like that. Chekov is serving on Pine's ship, a ship which apparently, Kirk got to see under construction when he doesn't appear to be any younger than he does when he's on the ship.... Heck, just Kirk meeting Pine at this point is a huge disruption of canon....
Thanks to Ron Salon for the signature banner!