Imagine a cyclist/motorcyclist at the back end of a queue. When driving, I have noticed that it is possible for the (my) brain to focus on the car in front of the cyclist whilst braking. This isn't normally a problem because as you get nearer the queue you will find focus on the two wheeler and apply more braking.
I think this might be something to do with the brain focusing on the larger more dangerous object. I've seen this referred to as the looming effect when the traffic is coming at you.
Suppose you were the kind of driver who drives fast, or was not paying enough attention, you approach the the traffic queue at speed and have to brake pretty hard. If you are committed to hard braking, it is then more difficult to brake even harder as your focus changes. Some will be over committed - and collide.
I too have been rear ended, luckily in a car, and I am quite paranoid about it now. On the bicycle or motorbike I try not to leave myself at the back of queue if fast traffic is approaching. I'll filter ahead or go way over to one side.