History of the Prophets and Kings
William Montgomery Watt mentions multiple possible reasons for the expulsion of the Banu Nadir in the forewordمقدمة of the translation of the seventh volume of al-Tabari's "History of the Prophets and Kings". First, the underlyingالأساسية reason for the expulsion of the Banu Nadir was the same as that of the Banu Qaynuqa, namely Jewish criticisms and hostility towards Muhammad and Islam fueling suspicionsتأجيج الشكوك among ordinary Muslims, keeping in mind that the attack came weeks after the Muslim loss of life in Al Raji and Bir Maona. Second, some members of Banu Nadir wanting to kill Muhammad, but Watt suggests that it is possible that this allegationsمزاعم was an excuse to justify the attack, adding that even if true, this was not the fundamental reason for the attack. Third, a chief of Banu Nadir gave information to Abu Sufyan during the barley-meal raid in 624.