If you slam the brakes in front of a car for no reason and they rear end you, you are most definitely at fault.

At least in my state, you're supposed to allow something like a minimum of 4 seconds or some length of space between you and the car you're behind. The argument is that the car rear ending you should have been far enough behind to stop.

Effectively, if you get into an accident with a car infront or to the side, you aren't at fault when the guy right behind you hits you too.