Roderigo and Iago have been enemies for a long time and are simply arguing about unimportant matters. They, in fact, always do this, and thus it isn't anything unusual.
Roderigo has been paying Iago to try to win the favors of Desdemona (Roderigo's crush). Iago hasn't been making any progress at all. In fact, Desdemona has recently married another man, Othello. This made Roderigo angry.
Roderigo owes money to Iago after a betting match had gone wrong terribly, and Iago had caught Roderigo (who had been avoiding him).
They were not arguing at all. They were merely pretending to argue.