Of the 700000, not 1 million, mandatory Palestinian Arabs who left in 1947-48:
The first 100000 who left right after the UN partition vote were the well-to-do who decamped to Damascus, Cairo and Beirut to wait out the war they knew was coming.
As that war heated up in early 1948 an additional 200000 left when told to do so by their leaders in Haifa, Jaffo and other towns. The Arab League's leader Azzam Pasha, had already announced a "war of extermination"
The real waterfall came after the battle of the strategic village of Deir Yassin on the road to Jerusalem, which Arab propagandists blew up as a massacre and mass rape of Arab women. Female and family honor trumped land as a value, and Jerusalem Arab leader Dr. Khalidi's strategy of using fake news of a massacre that never happened to get the Arab armies to fight Palestine's Jews blew up in his face (https://blogs.timesofisrael.com/deir-yassin-the-end-of-a-myth/):
"Deir Yassin was not the peaceful village many later claimed it to be, but a fortified village with scores of armed combatants. Its relations with the adjacent Jewish neighborhoods were troubled for decades and the Jews believed it to endanger the only road from Jerusalem to Tel Aviv, thus constituting part of the Arab siege of Jewish Jerusalem. Therefore, although later denying it for political reasons, the Jewish main militia in 1948, the Haganah, sanctioned the attack and later took part in it by means of its striking force, the Palmach.
A ten-hour fierce battle, in the presence of a civilian population, ended in the victory of Etzel and Lehi. No massacre took place. When the battle ended, the killing stopped. “I believe that most of those who were killed were among the fighters and the women and children who helped the fighters,” one of the Arab survivors was later to testify. Furthermore, the Arab villagers got an advance warning to evacuate the village, which 700 of them followed. The attackers took an additional 200 villagers prisoner and safely released them in Arab Jerusalem. Only 101 Arabs were killed, a quarter of them active combatants and most of the rest in combat conditions. The Jewish assailants also suffered casualties.
The Creation of the Palestinian Refugee Problem
For psychological warfare considerations, Etzel reported 200 Arabs killed, twice more than the actual number, enthusiastically adopted by the Palestinian leadership in Jerusalem, which increased it to 254 and added rapes and other gender-oriented atrocities. Hussein Khalidi, the senior Arab authority in 1948 Jerusalem, was of the opinion that, “We must make the most of this.” As his assistant Hazim Nusayba reported in a 1998 interview, Khalidi said “we should give this the utmost propaganda possible because the Arab countries apparently are not interested in assisting us and we are facing a catastrophe….So we are forced to give a picture – not what is actually happening – but we had to exaggerate.” Khalidi’s distortion of the facts failed to prevent catastrophe. Instead, it helped created one.
“Dr. Khalidi was the one who caused the catastrophe,” one of the Arab survivors ruled. “Instead of working in our favor, the propaganda worked in favor of the Jews. Whole villages and towns fled because of what they heard had happened in Deir Yassin.” The Palestinian leadership intended to exploit the affair to lay pressure on the Arab states to send their armies to Palestine to fight the Jews. The plan boomeranged. Following the rule that women’s honor comes before land, the moment the Palestinians heard about rapes they started to leave."
