The car in the rear isn't trying to pass. They're weaving around and gesturing for the pull over or stop from the looks of it.

Brandishing a weapon is maybe not the best idea, but I can think of much more hazardous courses of action. The gun may have had an empty magazine, a clear chamber, and the safety on...either way, it's obvious there was no intent to even aim towards the car. I'm not a gun enthusiast, I don't concealed carry (even though it's very easy in my state), and I don't hunt (while living in one of the best deer hunting counties in the nation). Hell, I'd even advocate for stricter penalties for offenses where a gun is fired. I do have quite a few guns I've inherited/had gifted from family, and I'm more than proficient with a rifle or pistol. It's hard to have an averse reaction to this because guns have become boring to me and don't carry much of an emotional charge when I see one. I was over the whole "holy shit, a gun!" the time I was 16. Now If a gun is being pointed at me/anyone else, can bet I'll have a reaction.

Having been a passenger in a car on the receiving end of some seriously over-reactionary road-rage, that shit can get heated quickly. People get angry and lose their minds. If this .gif turned out to be a genuine event, however unlikely that is, it ended as well as a road-rage situation where one party is weaving all over and looking for a confrontation could end. The instigator realized maybe they should reassess their battle-picking ability and chill the fuck out.