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.
Yes. This happens frequently in warfare. As you say, there are many different reasons why all those Arabs left Israel. To their credit, the Jews can point out that some 20% of the Israeli population is still comprised of Arabs, while there are hardly any Jews in the Arab nations (before 1948, some 750,000 Jews used to live in these nations.