Earlier quoted context omitted.
Humans in traffic are relatively easy to predict. They don’t just randomly brake for no reason.
I hear people talk about people brake checking all the time (whether they're the brake checkers or the person tailing). A buddy of mine was tailing a (non-Tesla) car a little too closely on his motorcycle and the driver deliberately brake-checked him and he wrecked. And that's just people deliberately driving erratically, never mind the people who are responding to debris, animals, people, etc darting into the road.
If both drivers only had basic insurance covering damages that aren't your fault, he got nothing and the other driver got paid for having his car damaged by a reckless driver that smashed into his car.
How does your friend even know it was without reason? Maybe a kitten or a squirrel crossed the road.