What is the connection between Shemitah and Har Sinai - bearing in mind that all the Mitzvos were said at Sinai?
Rashi and Seforno: To teach us that, just as Shemitah was given at Sinai (K'lalosehah) together with all its details (P'ratosehah), 1 so too, were all the Mitzvos given at Sinai together with all their details. 2
Ramban (citing the same Toras Kohanim as Rashi): When the Tana mentions "K'lalosehah", he is referring to the Pasuk in Mishpatim "ve'ha'Shevi'is Tishmetenah u'Netashtah ... ", whereas "P'ratosehah" refers to the rest of the Parshah. And it is the final Pasuk at the end of Bechukosai "Eileh ha'Mitzvos ... " that compares all the Mitzvos to Shemitah. 3
Rashbam: It teaches us that Shemitah was said at Har Sinai, before the Ohel Mo'ed was erected.
Why does the Torah insert the Mitzvah of Shemitah here?
Ramban #1 (citing Ibn Ezra): Because, when Hashem entered into a B'ris with Yisrael, 1 he taught them the Parshah of Bechukosai, 2 which it inserts here to connect it with that of the Arayos, in Acharei-Mos and Kedoshim, because, like the latter, 3 it is subject to exile from the land. 4
Ramban #2: Har Sinai in this Pasuk is referring to the second time that Moshe ascended the Mountain to receive the second Luchos, and that is when Hashem taught him about the Shemitah. 5
Ramban: Only He inserted Emor first, in order to mention the Shabbasos before the Shemitah.
The Ramban proceeds to present the time-table of Pesukim from after Matan Torah in Mishpatim till the current Pasuk. This is what he writes, beginning Parshas from Vayikra, following the erection of the Mishkan: Immediately, Hashem called Moshe and commanded him the Parshah of Korbanos ( in Vayikra, 1:1) and all of Toras Kohanim, which Moshe immediately taught to Aharon and his sons and to all of Yisrael. When he finished, he also told them that Hashem had commanded him on Har Sinai to explain to them the Shemitah and Yovel, and to enter into a (new) covenant with them on all the Mitzvos and Mishpatim with a curse and an oath. This covenant did not require the Shelamim sacrifices that the firs tone did; they did however, reaccept the original covenant with the oaths and the curses (mentioned in Bechukosai), as the Pasuk attests there (See 26:46), because Hashem pardoned them (for the sin of the Eigel) on these conditions, as the Ramban explained in Shemos 34:47. In fact, Yisrael reaffirmed the B'ris again under the same conditions at Arvos Mo'av (See Devarim 28:69).