The books with which I am the most familiar claim that while the Israeli authorities in 1948 had no formal policy for expelling Arab population, they had what might be called a broadly opportunistic policy for doing so, which in some cases was augmented by specific military plans.
Benny Morris cites a quote by David Ben-Gurion: “I support compulsory transfer. I do not see in it anything immoral.” (“Rightous Victims,” p.253)
According to Morris, at the beginning of the War of 1948, Arab populations fled either because of fear of violence, or because of actual attacks:
“The principle cause of the mass flight of April-June was Jewish military attack, or fears of such attack. Almost every instance - the exodus from Haifa on April 21-May 1; from Jaffa, during late April - early May; from Tiberias on April 17-18; from Safad on May 10 - was the direct and immediate result of an attack on and conquest of these neighborhoods and towns. In no case did a population abandon its homes before an attack; in almost all cases it did so on the very day of the attack and in the days immediately following…Haganah documents described ‘a psychosis of flight’ gripping the Palestinian population during this period. The echo of the slaughter on April 9 of the villagers of Deir Yassin, augmented by Arab atrocity propoganda regarding what had happened there, both reinforced and symbolized this.” ("Rightous Victims, p.255)
Simha Flapan, in his “The Birth of Israel,” says:
“It is true, or course, that many Palestinians left of their own accord…but hundreds of thousands of others, intimidated and terrorized, fled in panic, and still others where driven out by the Jewish army, which, under the leadership of Ben-Gurion, planned and executed the expulsion in the wake of the UN Partition Mandate…The balance is clear in the IDF intelligence estimates. As of June 1, 1948, 370,000 Arabs had left the country, from both the Jewish parts and the Arab parts conquered by the Jews. Jewish attacks on Arab population centers, particularly large villages, townlets, or cities, accounted for about 55 percent of those who left; terrorist acts by the Irgun and LEHI, 15 percent; whispering campaigns (psychological warfare) about 2 percent; evacuation ordered by the IDF, another 2 percent; and general fear, about 10 percent. Therefore, 84 percent left in direct response to Israeli actions, while only 5 percent left on orders from Arab bands. The remaining 11 percent are not accounted for in this estimate and may refer to those who left voluntarily.” (p.89)
These figures are as of June 1, 1948. At least the same number again fled later in the war. Both Morris and Flapan describe the expulsion of roughly 60,000 Arabs from the cities of Ramla and Lydda in July of 1948. And in the third stage of fighting in October and November of 1948, some IDF units enthusiastically expelled Arab populations, while some did not:
“…Israeli troops, both in the “Ten Days” in July and during Operations Yoav and Hiram in October-November 1948, were far more inclined to expel Palestinians than they had been during the first half of the war. In Operation Yoav, Allon took care to leav almost no Arab communities along his lines of advance. In Operation Hiram, in the north, where Moshe Carmel commanded the Israeli forces, there was confusion and ambivalence. Despite Carmel’s October 31 guideline “to assist the Arabs to depart,” some units expelled the villagers, others left them in place.” (“Rightous Victims,” p.257).
Based on these sources, a very significant portion of Palestinian refugees fled their homes either because of direct violence, or by immediate fear of violence, rather than because of more abstract reasons.