Albert Barnes Commentary Numbers 16:1

"Now Korah, the son of Izhar, the son of Kohath, the son of Levi, with Dathan and Abiram, the sons of Eliab, and On, the son of Peleth, sons of Reuben, took [men]:" — Numbers 16:1 (ASV)

Amram and Izhar were brothers . Thus Korah, the son (that is, descendant) of Izhar, was distantly related by cousinship to Moses and Aaron.

Although Korah was a Kohathite, and therefore from the division of the Levites with the most honorable charge, he probably regarded himself as wronged. This was because Elizaphan, who had been made chief of the families of the Kohathites (Numbers 3:30), belonged to the youngest branch descended from Uzziel (Numbers 3:27). Consequently, Korah took the lead in this rebellion.

Of the others involved, On is not mentioned again; he probably withdrew from the conspiracy. Dathan, Abiram, and On were Reubenites. They were probably discontented because the birthright had been taken from their ancestor (Genesis 49:3), and with it the primacy of their own tribe among the tribes of Israel.

The Reubenites encamped near the Kohathites (Compare to Numbers 2:25 and the plan), and so the two families were conveniently situated for taking counsel together.

One pretext for the insurrection was probably to assert the rights of primogeniture—on the part of the Reubenites against Moses, and on the part of Korah against the appointment of Uzziel.