Why does it say "Se'u She'arim Rosheichem..."?
Rashi (from Shabbos 30a): When Shlomo wanted to bring the Aron into the Kodesh ha'Kodoshim, the gates clung to each other and would not let it enter. Shlomo sang 24 praises to Hashem, but they remained closed until he said "Al Tashev Pnei Meshichecha [Zachrah l'Chasdei David Avdecha]" (Divrei ha'Yamim II, 6:42).
Radak: This is a metaphor, like "Yir'am ha'Yam u'Mlo'o; Ya'aloz Sadai v'Chol Asher Bo" (96:11-12). Today will receive a great honor - Melech ha'Kavod will enter!
Why does it say "Pischei Olam"?
Rashi (9): They are openings whose Kedushah is permanent.
Radak: Until now, the Aron went from place to place - from the Midbar to Gilgal, Shilo, Eretz Pelishtim, Beis Shemesh, Kiryas Ye'arim, Beis Oved Edom and Ir David. Now it will enter the place where it will stay l'Olam (permanently)! Similarly, it says "Machon l'Shivtecha Olamim" (Melachim I, 8:13). 'Sha'ar' is the lintel with the doorposts; 'Pesach' is what remains from the Sha'ar from the doors and outside. 'Pesach' implies that it is always open.
Malbim: They are the openings between the upper world and the lower world.
Why should "Pischei Olam" rise?
Malbim: Hashem is coming to dwell in the lower world. They are forced to rise, for He overrides nature as He desires. Also, they should rise amidst fear.
Why is He called "Melech ha'Kavod"?
Radak: It is because His Kavod dwelled on top of the Aron, between the Keruvim. We find "Asher Nikra Shem Shem Hashem Tzevakos Yoshev ha'Keruvim Alav" (Shmuel II, 6:2).
Malbim: All honor in the world, He is King over it, for everything is His, and His kingship rules over all honor.
