Why is the driver being held? You’ve got a witness affirming self-defense, the driver called it in and waited for the police, he is most likely a local with longstanding ties to the community. Why would you not release him on his own recognizance while being investigated? Does he have a criminal record or something?
Probably because the law in that state may say that if you have the opportunity to avoid the fight (by driving away) that you are wrong to shoot.
On the other hand, it is against the law in every state to leave the scene of an accident and the penalties are draconian in most states.
So the lawyers will get to sort it out.
As for me, my sympathies are with the truck driver. Unless he ran the light, and there is nothing here to indicate that he did, the bike rider was way out of line to start a fight.