And they didn't expel the Palestinians in 1947. The large wave of Palestinian refugees was created in 1948, when Arabs attacked Israel in an attempt to wipe it out.
It will be wrong to say that the Arabs were not expelled. Most of the migration was forced. But at the same time, I would say that the same would have been the fate of the Jews, had they lost the war.
Some Arabs were expelled during the 1948 war. Others left of their own free will, in order to make room for the Arab armies so that they could more easily kill Jews. Others ran away in fear of retaliation. There are many different reasons why all those Arabs left Israel during the 1948 war.
Of course there were clashes before the 1948 war as well. Through 1947 hostilities increased, and there were many cases of Jews being under siege and attack by Arabs, and Jewish militia moving in to aid them. This often meant removing Arabs that were standing between the militias and the Jews that were under siege or attack.