Not true, there are cases where you can rear end someone and not be at fault.
- They pulled out in front of you leaving you not time to avoid them.
- They change lanes in front of you, again leaving you no time to avoid them (assuming you’re doing the speed limit).
My guess, that accident was a combination of rubbernecking and improper lane change. The bike changed lanes without looking, and the van was staring at cop car on the side of the road.