If the driver of the lead car slammed on his brakes for no reason, he caused the accident.
Wrong again. One is required to follow at a safe distance so one can stop in time of a sudden event. Just because someone slams on their brakes, does not give you the right, or the excuse to rear end or blame them.
In addition, if you look on the street you can see painted on the street a warning of intersection/stop/crosswalk ahead.
The vehicle he rear ended was right at the limit line or very close to the intersection, which would mean if the they had the red light, the vehicle that was rear ended was braking for the intersection.
You must always follow at such a distance where if the car in front of you stops short, you can safely avoid them. Jenner had a trailer so he should have left even more distance. 999 out of 1,000 times the rear-ender is at fault. At least in my days of taking accident reports.