Why does it say "Neged Avosam Asah Fele"?
Rashi: Afterwards, when the Ketz came (refer to 78:9:1:1), they sinned more (verse 17).
Rashi (from Bereishis Rabah 92:2): Avraham, Yitzchak and Yakov came to the sea, to see how Hashem will redeem their children.
Radak: This refers to those born in the Midbar, who did not see Hashem's wonders in Egypt. They heard from their fathers that Hashem did wonders for them. Pele is a Klal; Hashem did many wonders for them!
Malbim: [They forgot His acts] even though He did wonders in the eyes of their fathers, who passed on to their children what they saw. Further, the wonders were revealed to all of Egypt - "l'Chol ha'Osos veha'Mofesim
Why does it mention "Sede Tzo'an"?
Radak: This is a repetition in different words. "Sede" is like Eretz - "Sede Aram" (Hoshe'a 12:13). "Tzo'an" is Egypt - "Lifnei Tzo'an Mitzrayim" (Bamidbar 13:22). It is the capitol of Egypt.
