Most of Numbers 32 is a negotiation between three parties with Moses acting as mediator: 1) Reuben and Gad, 2) The other 10 tribes, 3) God.

Reuben and Gad were camped beside each other in the wilderness, so it's no surprise that they asked for adjoining land in Canaan. I wonder if Reuben's former position as the firstborn left a lingering bitterness and contributed to their desire to be on the opposite side of the Jordan from Judah.

Dathan and Abiram were Reubenites. Were they hoping to set up a separate kingdom one day, perhaps? On the other hand, verse 2 seems to indicate that this was a populist movement rather than an idea of the tribal leadership.