What does "va'Yisga'ash va'Tir'ash" discuss?
Rashi: This does not refer to miracles that happened to David, rather, to Yisrael. 1
Radak: From here until verse 17 hints to eradicating the enemies of Yisrael. The noise of the land and Shamayim, darkness and thick cloud, coals, fire, arrows and lightning are all metaphors. Also "va'Yet Shamayim va'Yered" (verse 10) is as if He descended to eradicate them quickly; the same applies to verse 11.
Radak (Tehilim 18:8, citing R. Shmuel bar Nachmani in Shochar Tov): This refers to the interruption between one kingdom and another - "va'Tir'ash ha'Aretz va'Tachol Ki Kamah Al Bavel Machshevos Hashem" (Yirmeyah 51:9; Bavel will be destroyed).
Malbim (Tehilim 18:8): The earth opened its mouth and swallowed his enemies alive.
Radak (Tehilim 18:8): Rashi and the Targum hold that from here until verse 17 hints to miracles that Hashem did for Yisrael. "Va'Yisga'ash va'Tir'ash ha'Aretz" is the destruction of Sedom and Amorah. "Smoke came in His nose" (verse 9) refers to Pharaoh. "Hashem bent Shamayim and descended" (verse 10) refers to Matan Torah; also Shochar Tov says so. "Yar'em Min Shamayim Hashem" (verse 14) refers to hail that fell in the days of Yehoshua. "Va'Yera'u Afikei Yam" (verse 16) refers to splitting Yam Suf and the Yarden. David says (verse 17) that Hashem, who did all these for Yisrael, will do wonders also for me, and "send from above to take me." This is not the simple meaning of these verses (refer to 22:8:1:2).
Why were the foundations of Shamayim agitated?
Malbim (Tehilim 18:8): This is like the [volcanic] mountains of Ad, which open their craters at a time of noise, and are agitated with anger and burning fire.
What is the meaning of "Ki Charah Lo"?
Rashi: This is the reason for the start of the verse. When Hashem was angry [due to those who anger Him, the land erupted and made noise
