It is because of a social psychology principle called ‘social proof’.Let me give an example. Recently I wrote an answer What are the girls secrets that a boy shouldn’t know?. This answer sort of went viral. But wait, it started with a slow speed and it took the answer almost ten days to reach 1k upvotes.
You know how much time it took to reach 2k up votes from 1k up votes ? Three days!
Have a look at this screenshot.[pullquote]When more and more people find that the answer is liked by many more, it becomes easier for them to like it. This experience can be cross checked by many more authors.[/pullquote]

This is the power of social proof. When more and more people find that the answer is liked by many more, it becomes easier for them to like it. This experience can be cross checked by many more authors.
It’s all in the mind.First up vote and 1000th upvote are not same!
Source: What is the psychology behind upvoting answer on Quora after seeing a lot of upvotes?