Why do rabbits often have white hindquarters?
Apparently the most noticeable research on this subject of a rabbit's white behind has been done by Dr Dirk Semmann of the University of Goettingen. He proposes that 


  these spots actually confuse predators because of their very
  noticeable nature. By focusing on the bright spot, the would-be
  predator ignores the larger body of the animal. Then, when the rabbit
  executes a sharp turn, the spot disappears and the predator has to
  readjust to focus on the camouflaged coat, losing vital seconds
  (reference 1, reference 2).


It could be the same reason for the white behind of a deer. As to why deer flag their tails, there is a paper on the subject which lists 4 possible reasons for the behaviour. 

1) Flash behaviour

2) Alarm Signal

3) Cohesive signal

4) Detection signal

More can be read about it in this paper.