The lead up also does NOT show the victim threatening these agents in any way, with the gun or otherwise. Instead, they pushed a woman down and this guy tries to shield her and that's when they target him.
EDIT: another angle showing the run up: https://www.reddit.com/r/Leakednews/comments/1qlvt7t/video_f...
This has happened before. Once one shot goes off they all shoot at the suspect.