en_tn_condensed/isa/28/21.md

Yahweh will rise up

Yahweh preparing to act is spoken of as if he were sitting and then rising up.

Mount Perazim ... Valley of Gibeon

These refer to places where God miraculously defeated enemy armies.

he will rouse himself

"he will become very angry"

his strange work ... his strange deed

This work is strange because God is using a foreign army to defeat the people of Jerusalem rather than helping the people of Jerusalem defeat their enemies.