While most Muslima and Jewish tribes in Medina were agriculture based, the Jewish tribe of Banu Qaynuqa consisted mainly of craftsmen and blacksmiths, making them highly militarized. The tax-free marketplace, along with the Muslim's conflict with the Meccans (one of Banu Qaynuqa's main customer base) threatened Banu Qaynuqa's financial interests.
This led to Banu Qaynuqa violating the peace treaty they had agreed to in the Medinan Charter and secretly start colluding with the Meccans to incite hostilities against the Muslims.
Once Banu Qaynuqa began openly violating the peace treaty and began abetting the Muslim's enemies, there was a stand off between them and the Muslims, which led to Banu Qaynuqa's defeat and banishment.
Lecture notes available at http://www.why-quran.org/?p=474.
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