On what grounds was Hashem afraid that Yisrael may break their ranks and ascend Har Sinai, in spite of having already warned them in Pasuk 12?
Seforno: Since Hashem was about to speak to them firsthand, He was afraid that they would consider themselves on a par with Moshe Rabeinu, and ascend the mountain in order to join him.
Da'as Chachmah u'Musar 1 p.53: Even Kohanim at the highest level of closeness to Hashem are prone to want to show that they are more important. One may not take Chochmah for (to aggrandize) himself!
Oznayim la'Torah: To teach us that one needs to warn immediately prior to the act.
Man was created to enjoy the raidance of the Shechinah (beginning of Mesilas Yesharim). Ma'amad Har Sinai was the greatest revelation of Shechinah in this world. For such a spiritual Hana'ah, people were prone to do an Aveirah Lishmah. (PF).
Oznayim la'Torah: So great was the desire to see (even what was forbidden to see) that some people would break their ranks to see
Why does the Torah write "ve'Nafal Mimenu Rav" (and not "ve'Naflu