Difficult to do, but watch it again. Shaver reached behind his back with his right hand towards his waste band a second time at 3:44 an instant before the officer unloaded. The action is partially obscured by the officer's scope. Hindsight being 20:20, the guy was only trying to pull up his shorts, which kept falling down.
Because at that time, they were unsure of what/who else may have been around the corner in the hotel room that the individuals had just exited. The officers were responding to a report of someone pointing a rifle from that floor's hotel window. Advancing to where Shaver and his female friend were on the floor would have brought the officers closer to (and perhaps in the line of fire from) the possible threat from the hotel room before they had these two individuals secured. As it turns out, it in fact had been Shaver's rifle that had been spotted, and from that hotel room. It was Shaver's air rifle that he was showing off to friends, a rifle that he used in his job in pest control to shoot birds in warehouses.
I have two big takeaways from this video.
First, officers need to be better trained in how to diffuse extremely tense life-and-death situations. The officer's screaming and yelling at Shaver, who was clearly flustered and therefore having a difficult time thinking clearly, calmly, and following instructions, was not at all helpful. It only made it more difficult for Shaver to actually follow the instructions that were being given to him.
Second, people in general need to understand (but so many don't seem to) that when an LEO is pointing a weapon at you and giving you instructions, you need to follow those instructions explicitly. Do exactly what you're told and don't do anything other than what you're told, especially if that other action involves something that can remotely be perceived as threatening, such as suddenly reaching behind your back to your waste band to pull up your shorts after repeatedly being told to keep your hands up. For Christ's sake, let your shorts fall.
The whole thing is unfortunate. But I do believe the officer justifiably perceived a threat, even if that perception turned out to be inaccurate in hindsight.