You didn't say when exactlly in 1948. Pre state, first truce, second truce....
Anyway, three PODs I can think off.
Israel suffers more casualties in 1948-49 war so there is big drive for increased immigration to increase the numbers and replace losses.
Another is that after 1967 war Israel makes a peace with Jordan that gives them control over part of West Bank. That land becomes a part of Israel and there is drive to populate it, being promised land, heart of Jewish faith and so on.
Or in 1948-49 war Israel ends up with chunk of West Bank (with or without Jerusalem). Again moving to promised land, heart of Jewish faith etc.
I don't think it would be flood but a steady flow.