What did Moshe see?
Da'as Zekenim #1 and Rosh #1: He saw that the letters [of Aseres ha'Dibros] flew off the Luchos. Therefore, he grabbed them and broke them in front of Yisrael's eyes.
Da'as Zekenim #2 and Rosh #2: He saw that Yisrael were worshipping the Eigel. Refer to 9:17:2:2*.