I think it's a natural reaction to try to avoid what you see as an impending crash, but it makes no sense if doing that is going to result in another crash which might involve more cars or cause more injury. Braking is often not the best course of action to avoid crashing, but if you are not already aware of the other cars around you and whether or not you can move over sagely, then braking is about the only option you have. It should be sufficient, but in today's dense traffic almost nobody doesn't follow too closely, for to leave the proper space between you and the car ahead almost guarantees that some nut will move into that space. If you then slow to allow for that, it slows the traffic behind you who will now want to go around, and the whole process begins again in another lane which eventually will filter back into your lane in the space ahead of you making everything worse for everyone.
The root cause of most rear-end collisions is that the car behind was watching the car ahead, but wasn't watching further up the road to see the other cars further ahead slowing down which would have given them more warning that they'd need to be braking PDQ. Which goes back to the first lesson we all heard when learning to drive: "watch the car in front of you", which is not what you should be doing. You should be watching everything in front of you including that one car. And you should be as aware as you can of the cars beside and around you enough to know if you can move over safely in an emergency if that is a better choice than braking. All of this has to be done ahead of time and constantly, for there isn't going to be enough time to do it when things begin to go sour. Pilots are taught to not pay attention to what is happening right now, but to already be handling that by having looked and thought ahead because most planes do not react strongly to control input like cars do- it takes time to get on the heading and attitude you want or to adjust your speed. You simply must think ahead and be doing the right thing already or you're going to crash, which in a plane often means you die in that crash. That kind of thinking should be applied to your driving. Everyone's driving. And anyone who cannot or will not do this should not be driving. None of us is perfect, but all of us can do better if we pay adequate attention and try. Unless there is an adequate reward for doing this or a severe penalty for not, then most people aren't going to bother putting this much effort into their driving because there is no reason to.
Phil