Sounds like it might be due to inter-agency rivalry. Maybe this guy did something ATF didn’t like (apply the law logically, or something equally stupid) and a couple of them vowed they’d nail him for something, someday.
This case demonstrates, once again what FBI really means - Fumbling, Bumbling, Idiots.
Gun grabbing, collectivist idiots, at that.
The victim must have made an enemy somewhere in the FBI.