Some people don't have a clue what traffic is all about. They really don't. Well, perhaps you do, or it's different in your part of the world, but...
The problem splits up in several parts. One is speed. Many people don't seem to understand that when they make their car move at 80mph/120kph, that they are moving fast. What? Oh, of course, everyone moves fast, so there's no problem. Really? Speed kills. Accidents happen. People die.
Then there are the people who are not moving so fast. They putter on with 55mph/90kph, and decide they want to overtake the truck they've been behind for a while. Their problem? No idea of speed and traffic. They move forward, the people in the other lane move forward as well, so when there's an opening on the next lane that will hold their car, they move their car into that opening. The speed difference between them and the other lane - neglect that. 25mph is just a number, let's count on it that the other person's car has great brakes, that the other person in the car reacts very fast, and then there's no problem. The fact that there is probably a long string of cars behind that other person's car? Ignore that. As long as they are in the other lane, puttering along at almost 60mph now.
I really can't believe the idiocy that I see happening on the roads these days, and I spend a lot of time (far too much) on the roads. Not many people seem to think any more, seem to understand what this whole car game is about. It's not a race track, people, even when it looks that way because of the speed freaks. It's deadly out there.