What does the Torah mean when it writes "Al Tachrisu ... "?
Seforno: It means that they should not allow the Bnei Kehas to apply a policy of 'First come, first served', when coming to carry the holy vessels, because then, in their eagerness to perform the Mitzvah, they would push each other and desecrate the Kodesh, thereby resulting in their deaths. 1 But rather that Aharon and his sons, when coming to cover the vessels, should designate each ben Kehas to the specific task that he was to perform.
Seforno: As occurred in connection with the T'rumas ha'Deshen, when one Kohen pushed another Kohen off the Mizbe'ach, as the Gemara relates in Yoma, 22a.
Why is Kehas called "Shevet"?
Moshav Zekenim citing R. Yehudah ha'Chasid: Just like a Shevet that camps in the south is forbidden to camp in the north, so the families of Levi may not switch their order of encampment.