Before 1948, before any war or arab invasion, Israel expelled 300,000 Palestinian Arabs. THAT is the reason the arabs declared war:

Almost half of this figure (approximately 250,000–300,000 Palestinians) had fled or had been expelled ahead of the Israeli Declaration of Independence in May 1948,[22] a fact which was named as a casus belli for the entry of the Arab League into the country, sparking the 1948 Arab–Israeli War.[23]

In the period after the war, a large number of Palestinians attempted to return to their homes; between 2,700 and 5,000 Palestinians were killed by Israel during this period, the vast majority being unarmed and intending to return for economic or social reasons.[24]

https://en.wikipedia.org/wiki/Nakba#Components

And why did they leave? Well according to IDF intelligence, it was because of of Jewish attacks:

Undoubtedly, as was understood by IDF intelligence, the most important single factor in the exodus of April–June was Jewish attack. This is demonstrated clearly by the fact that each exodus occurred during or in the immediate wake of military assault. No town was abandoned by the bulk of its population before the Haganah/IZL assault... The closer drew the 15 May British withdrawal deadline and the prospect of invasion by Arab states, the readier became commanders to resort to "cleansing" operations and expulsions to rid their rear areas.

https://en.wikipedia.org/wiki/Nakba#Components

Haganah Intellegence corroborates this:

At least 55% of the total of the exodus was caused by our (Haganah/IDF) operations. To this figure, the report's compilers add the operations of the Irgun and Lehi, which "directly (caused) some 15%... of the emigration". A further 2% was attributed to explicit expulsion orders issued by Israeli troops, and 1% to their psychological warfare. This leads to a figure of 73% for departures caused directly by the Israelis. In addition, the report attributes 22% of the departures to "fears" and "a crisis of confidence" affecting the Palestinian population. As for Arab calls for flight, these were reckoned to be significant in only 5% of cases..

And of course, some prominent historians say the same:

Ilan Pappe:

According to Ilan Pappé, the Zionists organised a campaign of threats,[18]: 55  consisting of the distribution of threatening leaflets, "violent reconnaissance" and, after the arrival of mortars, the shelling of Arab villages and neighborhoods*

Steven Glazer:

In this sense, Glazer[33] quotes the testimony of Count Bernadotte, the UN mediator in Palestine, who reported that "the exodus of the Palestinian Arabs resulted from panic created by fighting in their communities, by rumours concerning real or alleged acts of terrorism, or expulsion. Almost the whole of the Arab population fled or was expelled from the area under Jewish occupation."*

Edgar O'Ballance

Israeli vans with loudspeakers drove through the streets ordering all the inhabitants to evacuate immediately, and such as were reluctant to leave were forcibly ejected from their homes by the triumphant Israelis whose policy was now openly one of clearing out all the Arab civil population before them... From the surrounding villages and hamlets, during the next two or three days, all the inhabitants were uprooted and set off on the road to Ramallah... No longer was there any "reasonable persuasion". Bluntly, the Arab inhabitants were ejected and forced to flee into Arab territory... Wherever the Israeli troops advanced into Arab country the Arab population was bulldozed out in front of them.[46]

https://en.wikipedia.org/wiki/1948_Palestinian_exodus