What fire consumed the young men?
Rashi: His Evrah (anger).
Rashi citing Shocher Tov: This refers to Nadav and Avihu 1 .
Radak: This is a metaphor for war - "Ki Esh Yatz'ah me'Cheshbon" (Bamidbar 21:28). Malbim - in a war of a Mitzvah, even a Chasan leaves his room and a Kalah leaves her Chupah 2 (Sotah 44b).
What is the meaning of "Lo Hulalu"?
Rashi: [The virgins] were not crowned with a canopy and Chupah; the young men died in the war. Hulalu is like Hilula (wedding) in Arame'ic. Radak - had they lived, they would have married the virgins. We find that Chupah is called 'Beis Hilula' (Kidushin 32b), for they are Mehalel (praise) the Chasan and Kalah.
Radak (105:5): They did not rejoice 1 .
Radak (105:5): Even though Chupah is called 'Bei Hilula', we can say that Hilul is an expression of praise.