Sforno on Leviticus 10:1/ספורנו על ויקרא י׳:א׳

1ויקחו שני בני אהרן איש מחתתו, they thought that just as after the daily communal offering which was accompanied by the manifestation of G’d’s Presence as we know from Exodus 29,42 עולת תמיד לדורותיכם פתח אהל מועד לפני ה' אשר איועד לכם שמה, “the regular communal burnt offerings for your generations in the presence of the Lord, where I will manifest Myself there,” there would come the incense, so it would also be in order on this occasion to present a new incense offering honouring the manifestation of the Lord to the whole people and in honour of the heavenly fire having descended.

2This is why they offered it in the sanctuary, i.e. 'לפני ה, on the golden altar. The Torah had stipulated that no foreign incense [nor other unauthorised offerings. Ed.] was to be offered on that altar (Exodus 30,9). Even assuming that the golden altar would qualify for additional incense offerings when a specific command to do so would be issued, the sons of Aaron sinned by doing this now and not having consulted with their mentors. This is why the Torah stresses

3אשר לא צוה אותם, “which He had not commanded them (to do).”