Why did Moshe not mention any sins of Yisrael prior to Matan Torah (refer to 9:8:1:1), except for that of Masah?
Why did Moshe mention the sins of Tav'eirah, Masah, and Kivros ha'T'avah here?
Ramban: It is because testing Hashem is a grave sin, as the Torah states in 6:17. It mentions also the Meraglim (verse 23), which was a very severe sin, to conclude "you have rebelled against Hashem."
Seforno: Because, although they saw how, when they angered Hashem, it caused the Luchos to be broken and Moshe had to pray for 40 days to repair the damage, it did not deter them from grumbling at Tav'eirah and at Masah 1 and from sinning at Kivros ha'Ta'avah.
Riva citing R. Elyakim: Yisrael were worthy to be wiped out at these places, just like for the Egel. For each of them, Moshe needed to pray for them for 40 days
The sin of Masah was before the Luchos were broken! Perhaps he holds like the opinion that Masah is the same as Mei Merivah, and it was after Miryam died (refer to Bamidbar 14:22:151:3, 14:22:151:3**. - PF)
Since he explains that this is why they were inserted here, presumably, the same applies to Chet Meraglim in verse 23. I find all this difficult. Masah was after the Manna fell, i.e. after Iyar 16 (Shemos 16:1), less than three weeks before Matan Torah, which was given amidst love. Was Moshe still praying that Hashem not destroy them?! Also, "Arba'im Yom v'Arba'im Laylah" implies constantly. Moshe was ascending and descended Har Sinai, relaying Hashem's words to Yisrael and vice-versa, and then ascended to receive the Torah! Tav'eirah and Kivros ha'T'avah were after we left Har Sinai on Iyar 20 (Bamidbar 10:1,2), less than 40 days before sending the Meraglim on Sivan 29! It is hard to say without a solid source that all of these are written out of order, unlike many Midrashim Also after Chet ha'Meraglim, the verses imply that Hashem decreed that same day that they die over the course of 40 years in the Midbar! (PF)