Car change lanes on highway

Is there a law against doing this? How would I explain this? Do you call it a law or a rule of driving?
Please reword to make more sense.

The car changed lanes too close to the car behind it. You don't change lanes with the car behind you this close. Wait to see the whole car in your miror before going into his lane. Car A wanted to change lanes from lane A to B. There was a car in lane B not far enough behind car A when it changed lanes causing the car behind to have to brake.