Hundreds of thousands of Palestinians didn't get up one day and move fifteen miles down the road because they all felt like going on a picnic at the same time.

Interestingly, the term "ethnic cleansing" wasn't coined until the 1990s. The 20th century push for ethnic self determination came with an understanding that people would have to be moved to form concentrated and contiguous populations that could self determine. Millions of people were forced to move in the 1940s to establish India, Pakistan, and a more Slavic Soviet Union.