Rashi #1: It belongs to the rebukes that comprise the current Parshah ¹ - based on the following episode hinted here: When Aharon died on Hor ha'Har, and the Ananei Kavod disappeared, Yisrael, threatened by the king of Arad, ² appointed a leader to return to Egypt, ³ and traveled back eight encampments to B'nei Ya'akan, ⁴ and from there to Moseirah. At that stage, the B'nei Levi fought with them and brought them back to Gudgod ⁵ . During the battle, many families on both sides were killed. ⁶ Whilst in Moseirah however, they attributed the many deaths to the fact that they failed to eulogize Aharon, so they eulogized him there and then. The Torah writes "Sham Meis Aharon", for due to the great Hesped there, it seemed as if Aharon had died there.

Rashi #2 (in Pasuk 7, citing Vayikra Rabah): To teach us that the death of Tzadikim in the eyes of Hakadosh-Baruch-Hu is akin to the breaking of the Luchos.

Rashi #3 (in Pasuk 7): To teach us that when they declared "Nitnah Rosh Venashuvah Mitzraimah" (after Aharon died), to separate from Hashem, it was as severe as the day that they made the Golden calf.

Rashbam: In answer to the potential query on Moshe - that Aharon died, so his Tefilah for him did not help, Moshe is reminding them here that the Golden calf was at the beginning of the forty years, and Aharon did not die until the fourtieth year, after they traveled from Be'eros B?nei Ya'akan, where they had arrived from Moseirah, and they traveled to Hor ha'Har [and he died there].

Seforno: 'Despite the fact that they saw how the Tefilah of a Tzadik (Moshe) shields over the generation - in which case they should have mourned Aharon's death, some of them - or even most of them, whilst pasturing their flocks in the desert, made their way to Moseirah to look for water and grazing-ground for their sheep. Yet when, whilst they were there Aharon died, they did not return to mourn for him or to participate in his burial. Nor did they take the trouble to pay their respects to Elazar, who succeeded him; but traveled straight from there to Gudgodah.'

Da'as Zekenim and Hadar Zekenim: According to the simple meaning, Moseirah and Be'eros B'nei Ya'akan are not synonymous with the Moseiros and B'nei Ya'akan mentioned in Bamidbar 33:31. ⁷ Hor ha'Har was between Be?eros B'nei Ya'akan and Moseirah, and when Yisrael left Be'eros B'nei Ya'akan on the way Moseirah, they came to Hor ha'Har, and Aharon died there.