If you need to ask the question whose fault it was... that is, you're not 100% certain that it was entirely the other guy's fault, and that you can prove it to a jury if necessary... it's at least partly your fault. Even if you are certain, it's still probably at least partly your fault.
It would be the car who has driven past, then reversed into you.
it is the person coming out of the parking space
If you were both backing out at the same time, then it will be considered joint fault, sometimes called contributory negligence. That means both drivers failed to notice the other driver.
Changing lane should be at fault because the driver should be able to control the car against hitting another car. The changing lane driver only has a case if it is illegal to use the side road.
Yours.
If "Car A" is parallel parking, it has the right of way over "Car B". If the parallel park was indicated properly by "CAR A", then "Car B" should not have been in a position to pull into space behind "CAR A" as "CAR B" should have hung back as soon as the paralell park was indicated by "CAR A".
It is their fault.
In all crashes there is an underlying cause or driving error. The person who made the error and presumably broke the law is at fault. In virtually all head on crashes some one is on the wrong side of the road, from there we need to know why, if they justed oopsed, the person on the wrong side is at fault. If there was another crash that forced the car into the wrong lane caused by a third car the third car would be at fault for the whole mess. And so on.
Your question is neither clear or definitive. How ever the driver of the car that drove into the rear of another car is at fault. I assume that the car that was speeding a bit was not involved, you do not include this vehicle in the accident, but he has no culpability.
Its the person that hit the car coming in to the parking lot because probably the bad car was speeding. :)
The person in reverse is most likely at fault unless u-turns are illegal where you did it.