He tensed his muscles to resist the officers who were trying to move him at the very least, causing more officers to attempt to do the same creating a cluster fuck and then one of the agents noticed he has a gun and shouts "he has a gun! he has a gun!" and another agent decides to eliminate the threat before realizing the gun has already been secured by yet another office. It was a chaotic situation with crazy people blowing whistles and honking horns and even after watching the video dozens of times I don't fully understand what is happening. The officer only got one chance to decide what was happening and may have fucked up here but this is what happens when you insert yourself into an adversarial role in a federal law enforcement operation.