On what grounds was Hashem afraid that Yisrael may break their ranks and ascend Har Sinai?
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 Chachmah for (to aggrandize) himself!
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)
Why does the Torah write "ve'Nafal Mimenu Rav" (and not "ve'Naflu
Rashi: Because if even one Jew falls, it is as if many fell.