1)
Why does it say that Hashem opened His storehouse?
1.
Radak: This is a metaphor - it is so they will know and understand that Hashem fights them.
2.
Malbim: The war that they fought against you was with Hashem's Kelim of anger. This corresponds to their taking Hashem's Kelim (of the Mikdash) for their feast. "Im Hashem Hisgaris" (24) hints to this.
2)
Why does it say "Ki Melachah Hi la'Shem"?
1.
Malbim: This task is attributed to Hashem, for He destroyed it, and you are the enemies.
