A Fire That Was Not Commanded

Rabbeinu Bachya explains the episode simply, according to pshat. Nadav and Avihu were meant to light the incense with fire taken from the mizbeach (altar). Apparently they used another source of fire, and this was the strange fire. The Torah also stresses that they were not commanded to bring it. They acted on their own, doing what they thought was right without first consulting their father Aharon and their uncle Moshe, the established authorities. Some might see such initiative as a good thing, but it is evident that Hashem did not.