1)
Why will they be on mountains?
1.
Radak: To be protected until the enemy troops will pass. "El he'Harim" is like Al he'Harim.
2)
What is the comparison to "Yonei ha'Ge'ayos"?
1.
Rashi: Doves gather in canyons and coo 1 there. [The remnant] will moan on the mountain to which they will flee - each will moan over his sin, like cooing doves.
2.
Malbim: The remnant who flee there will be like doves that are normally in canyons and crevices. They are not normally on mountains. Due to this, all coo. So each man will moan that his plight is "ba'Avono".
1
Radak: They make a sound like lamenting.
