Morris listed the reasons for why Palestinians left in 1948. Number one and two on the list were direct expulsion (or massacre) by zionist/Israeli forces and fleeing before the zionist forces actually got there (presumably to avoid being massacred).
Morris in
Revisited: "In examining the causes of the Arab exodus from Palestine over 1947–1949, accurate quantification is impossible. I have tried to show that the exodus occurred in stages and that causation was multi-layered...What happened in Palestine/Israel over 1947–1949 was so complex and varied, the situation radically changing from date to date and place to place, that a single-cause explanation of the exodus from most sites is untenable...At most, one can say that certain causes were important in certain areas at certain times, with a general shift in the spring of 1948 from precedence of cumulative internal Arab factors – lack of leadership, economic problems, breakdown of law and order – to a primacy of external, compulsive causes: Haganah/IDF attacks and expulsions, fear of Jewish attacks and atrocities, lack of help from the Arab world and the AHC and a feeling of impotence and abandonment, and orders from Arab officials and commanders to leave. In general, throughout the war, the final and decisive precipitant to flight in most places was Haganah, IZL, LHI or IDF attack or the inhabitants’ fear of imminent attack."
The Arab armies intervened on May 15th AFTER months of expulsions and massacres by the zionists. It is estimated that about 300,000 (out of the total of 750,000) were expelled and hundreds of villages destroyed during this period.
According to Morris, up until April there were very few cases of coerced expulsion or destruction of villages - "in the war’s first four months, between the end of November 1947 and the end of March 1948, there were no preparations for mass expulsion and there were almost no cases of expulsion or the leveling of villages". He then argues that "as a result of Arab belligerence and the Yishuv’s sense of siege, fragility and isolation", a harsher policy of transfer came into play from April onwards.