He reached his arm back as he was turning around like anyone on a bike would do when a cop is giving him instructions from behind. Not trying to start an argument here, but the guy's arm/hand never made a motion like it was going to come up with a gun.
Remember also, that the angle of view is not the same as what the officer was seeing. He had to be a good 3-5 feet further to the left, which would have masked that arm, that much more.
(I'm not claiming this is a 'good' shooting, by any stretch.... but I can understand adrenaline, poor sight lines, high stress, and a very very quick sequence of events.)
No one has mentioned if there were any priors on either rider. No one has mentioned how long this pursuit had gone on prior to this video. No one has described any other actions by any party that may play into the LEO's threat assessment. How much meth-amphetamine use is common, where this occurred?
If he's already pulled a bead on the seated rider, and the other bike goes out of his view.... and then this guy does something with his hands that the LEO cannot see? I can understand where the rider's actions were perceived as 'hinkey'..... and I can also understand his innocent intent; if it was that.
Is the shooting justified? Almost certainly not, in the view of whatever jury is going to judge this. Can I understand how it happened? Yes.
No matter the bullet proof vest, no matter the badge, or the gun, or the cruiser, and the sirens and lights. The LEO has the right to go home at the end of his/her shift unharmed. They are trained to escalate to the next level of force required to maintain control of the situation. Gun comes out, 'center mass' is the only option. It's somewhat amazing to me, that he didn't empty his clip to be honest. Darned unfortunate that this LEO was proficient enough to hit center mass, and sever this guy's spine. 3" either side, and you're talking about a punctured lung.