Hashem agreed not to destroy Yisrael before Moshe descended (Shemos 32:14). Here it implies that Hashem was angry "Lehashmid Eschem" in the 40 days after Moshe descended!
Ramban: Despite the fact that Hashem had already retracted from His initial threat to wipe out K'lal Yisrael, Moshe found it necessary to ascend Har Sinai to pray on their behalf to protect them against the wrath and the rage of Hashem. 1
Targum Yonasan: When, following the sin of the Golden Calf, Hashem sent down five angels - Af, Cheimah, Ketzef, Mashchis and Charon - to destroy Yisrael, Moshe roused the Avos from their graves to pray for them. They succeeded in eliminating the latter three, and now, with his Tefilos, Moshe eliminated Af and Cheimah. 2
This refers to other Aveiros for which Hashem thought to destroy Yisrael (in verses 22-23). Refer to 9:22:2:3
I.e. the wrath and the rage that were aroused before He agreed not to destroy them. This is fine according to Targum Yonason, that Af and Chemah are damaging angels (refer to 9:19:1:2). Da'as Zekenim, Hadar Zekenim, Rosh (all on verse 18) say that Hashem agreed not to destroy Yisrael, but the sin was not pardoned; Moshe now requested pardon of the sin.
Targum Yonasan: Whom he subsequently buried in a pit in the land of Mo'av, by means of the Great Name of Hashem (The question remains however, why do Af, Cheimah and Ketzef Gadol reappear in 29:27? - EC)
What does "Gam ba'Pa'am ha'Hi" mean?
Hadar Zekenim: Just like the first time, Hashem agreed not to destroy Yisrael, now He agreed to pardon their sin.