Original: On May 1, 1921, the Jaffa Riots erupted and an Arab mob killed dozens of Jewish residents. In the wake of this violence, many Jews left Jaffa for Tel Aviv, increasing the population of Tel Aviv from 2,000 in 1920 to around 34,000 by 1925.
Simplified: Tel Aviv was part of Jaffa at first, but it became a separate town in 1921.