What do we learn from "Lo Yasuru Mimenu"?
Yoma 72a: This is a Lav. One may not (Rashi - ever) remove the poles of the Aron.
Riva citing R"M of Kutzi: The poles should fit tightly into the rings, so when those carrying it ascend a mountain 1 , the Aron will not slide down towards those in back, and when they descend, it will not slide down towards those in front.
It says that the poles of the Aron cannot be removed. Bamidbar 4:8 says "v'Samu Es Badav." This implies that they were removed!
Yoma 72a: The poles may slide through the rings (so they are mostly in front or mostly in back), but they are thick at both ends, so they cannot be removed (Riva). 1
Hadar Zekenim: The primary Avodah is through Aharon u'Vanav. Once they inserted the poles (Bamidbar 4:8), it was forbidden to remove them. Here, Moshe inserted them; there was no Isur to remove them.
Pane'ach Raza: When the Aron rested, the poles were mostly in front, and jutted into the Paroches. When they carried it, they needed to center the poles and make it fit tightly, so the Aron will weigh equally on the people in front and in back.