"Head jerk", "stutter step"; looks like the normal effects of recoil to me. And the officer did not "immediately holster" his weapon; he held what looks to me like a shooting stance for about a second after firing. As for not firing two shots: no target after the first shot, Thomas immediately dropped back into the vehicle.
To drive it home, Officer Feaster concealed the event, even if only by omission. Only when he realized someone was going to be sent to the cantina to investigate, and he realized he would be exposed anyway, did he finally admit he "might" have shot Thomas.
I get a sense that there was prior contact between Officer Feaster and Thomas. This looks to me a lot like an "This is the last time, asshole," shooting. Otherwise, why even draw the gun? All Thomas was doing was lifting himself out of the vehicle; and with both hands, no gun threat. Feaster was five steps away, it wasn't like Thomas was likely to escape. Even if he did manage to run, the police had the vehicle; he wasn't getting away. Given the circumstances, I don't see any reason even to draw.
But if Thomas was already known to Feaster--a prior, annoying history--then an irritated spur-of-the-moment draw makes sense. Even if we buy the DA's argument that the actual shooting wasn't intentional.
I'd give a lot to know what was said as Feaster was looking down into the car, right after the shooting. I suspect that would tell us a lot, one way or the other.