How is the effect of current vote scores on people's votes interpreted in psychology?
I think you can find an answer from the field of social psychology. The most relevant theory is the social proof, the tendency to immitate the behavior of other people.


  Social proof (also known as informational social influence) is a
  psychological and social phenomenon where people assume the actions of
  others in an attempt to reflect correct behavior in a given situation.
  
  ...
  
  Social proof is one type of conformity. When a person is in a
  situation where they are unsure of the correct way to behave, they
  will often look to others for clues concerning the correct behavior.
  When "we conform because we believe that others' interpretation of an
  ambiguous situation is more accurate than ours and will help us choose
  an appropriate course of action", it is informational social
  influence.