That is wrong in so many levels. If the car in front of you has to stop suddenly for any reason, it is your job to be far enough behind to be able to react and stop. Slowing down to turn or suddenly braking for some reason doesn’t justify rear-ending. Basically it’s only the driver in front’s fault if they are changing lanes unexpectedly, deliberately trying to cause an accident, or in reverse. Otherwise the driver behind has to stay off their ass and pay attention.

As I originally said, legally it is almost always considered the fault of the car behind if they hit you. However in the real world the actions of the car in front can increase the probability of getting hit. Most times in traffic people cannot follow far enough behind that they have enough reaction time should the car in front do something suddenly or unpredictable. If you allow the theoretical reaction space in many areas you will have multiple people pulling in front of you into the empty space. That’s the way it was driving in San Diego for example when I lived there. To make up for the tight spacing an experienced driver will look not just at the car in front of them but at the conditions in front of that car so they can know when to expect the car in front of them suddenly slowing or stopping. But if that car fails to signal an upcoming turn that can result in an unexpected sudden slowing or stop, or if they jam on the brakes because they got to close to car in front of them. Just consider the odds - Which if the two was the experienced driver, the driving examiner or the one who had been tested? While it is possible that the examiner is a poor driver or was distracted. It’s also very possible that OP did something that contributed to the accident. You will note they gave zero details about what happened before they were rear ended. It may seem satisfying to assume the driving instructor was a poor or distracted driver and that the presumably young driver who had just taken his driving test was blameless in this accident. But that’s not likely the full story.

I have found that new drivers from their tests are much more cautious at driving to start, and experienced drivers can't anticipate what a new driver would do over an experienced one. It's the whole debate of why people hate autonomous cars, because they don't drive like humans do; they drive like they're programmed. Hence why they're hit so much! The guy behind in this case should have kept a safer distance, as I highly doubt OP was brake checking him or really driving that weird other than braking earlier to slow down sooner. Test-ees still had to go through dozens of hours of practice to get their pass, it's not like they walked in with little to no experience, did it AND passed. That seems more unlikely than OP being at fault.