This article in Wikipedia provides a description of the nature of and numbers assoicated with the Palestinian exodus in the time period of the founding of Israel.
http://en.wikipedia.org/wiki/Palestinian_Exodus
In particular look towards the middle of the article for these topics:
First stage of the flight, December 1947 - March 1948
Second stage of the flight, April 1948 - June 1948
Third stage of the flight, July 1948
Fourth stage of the flight, October 1948 - November 1948
The aritlce also spends a little time debunking the myth that the exodus was instigated by Arab leaders. This myth has been debunked several times by a variety of people but that doesn't stop it from being repeated as fact even today.
The bottom line here is that Palestinians left their homeland for a variety of reasons:
voluntarily because of fear of impending war.
voluntarily because of threats of massacre or terrorism
involuntarily by forced ejection
voluntarily, sort of, when the peasant lands were purchased and the zionist kicked the peasants off land they had farmed for generations.
The simple fact is that Israel at the time of its creation was lead by people that wanted to found a Jewish state. This is the stated purpose of the zionist leaders of the time and it is documented over and over by their statements and actions.
There is no question that the zionist leaders had concerns about the indigenous population but there is also no question that concerns for the indigenous population were secondary to their main goal which was to found a Jewish state. Today, there is a never ending stream of obfuscation and misdirection to turn the nature of the founding of Israel into something that fits nicely with modern notions about morality and equal rights for people regardless of race or religion. The fact that actual history doesn't jibe with this seems to offer little impediment to the effort.