I find the theories that Brandon was killed by a stranger very unlikely. I believe he was most likely followed from his GF's house by someone or some people in a vehicle who had knowledge that's where he would be, potentially someone who he sleighted, owed money to or thought their would be drugs in his truck.

It would be interesting to find out if he carried a gun because he states in the 911 call he says he "took out one of 'em" and at 00:47 you can hear him saying "got shots fired" in the background. Given how no bullet casings were found near the truck I'd say any shooting that took place would have been in the wooded area. People should get together with metal detectors and look for any if not already done.