1)
Why does it say "mi'Tziyon"?
1.
Radak: This is like it says in Yo'el (4:16) "va'Shem mi'Tziyon Yish'ag umi'Yerushalayim Yiten Kolo." It is as if His voice comes from there to call them to Yerushalayim.
2.
Malbim: Hashem will make His Shechinah appear from Tziyon. From there He will judge the flatterers and reveal their evil.
