What does "k'Vala es ha'Kodesh" mean?
Rashi: It means "when they come to cover the holy vessels by placing them into their respective bags. 1
Ramban: It is a La'av against stealing a holy vessel, which is punishable by Misah bi'Yedei Shamayim and is subject to 'Kana'im Pog'in bo' (a zealot who catches him in the act is permitted to kill him on the spot).
Ramban (according to Kabalah): It means when the Shechinah is absorbed into the Kodesh Kodshim in Heaven. 2
Rashi: As described in the previous Pesukim.
Ramban: When Aharon and his sons took down the Paroches, the Kavod, Yoshev ha'Keruvim (the Shechinah), which was now in full view, returned to its former place in Heaven.
Because when something is consumed it becomes invisible.
Why would merely seeing the holy vessels being covered cause the Bnei Kehas' to die (via a Divine fire - Targum Yonasan)?
Seforno: Refer to 4:18:1:1.
Ramban, citing Ibn Ezra: Because anyone who looks at the Aron is Chayav Misah. 1
Rashbam: Because when they covered them, the Shechinah appeared. 2