Ah, this happens in all GTA games. People always drive and walk like they're too high to care about consequences. This type of chaos can happen in a variety of ways.
- Cars can hit other cars for no reason, because nobody ever bothers to make NPCs drive like most people would. When a collision occurs, nobody ever just pulls over and exchanges insurance information. Instead, drivers have a chance of panicking and floor it trying to get away which causes chain reactions, or they'll get their road rage on and try to murder the owner of the car that hit them. This kind of thing can snowball.
- When cars hit people, sometimes those people get mad and try to start a fight with the guy that hit them. Fights can leave idle cars sitting in the street; see the next point.
- In most city sandbox games, if there's an empty car or other obstacle in the road, non-moving drivers will get impatient and eventually attempt to go around whatever's blocking them at all costs. They typically drive onto the sidewalk and break things, hit people, and generally start driving like shit. If they can't get around whatever's blocking them, they often attempt to just ram it hoping to get through. This obviously causes problems. All it takes is one car parked in the middle of the road to eventually cause urban chaos, even if takes so long for that one car to get bumped enough that it explodes.
- Cops have a short fuse. Anyone running over a cop or crashing into a cop car is the latest candidate for police brutality. Cops can sometimes care enough to do something about people starting fights in the streets or running over other people, typically adding gunfire and setting everyone off.
- When people hear gunfire or explosions, people start to panic and floor the gas pedal and hit things without a second thought, even if it means smashing into things and being unable to go anywhere, or even burning cars set to explode. This is what happens in the video.
It's always funny unless one of these panicking fools happens to run over you. That hurts a lot and just makes you want to shoot everything.