Why leap ye, ye high hills? this is the hill which God desireth to dwell in; yea, the LORD will dwell in it for ever. Why leap ye, ye high hills? - "God said, Why leap ye, ye high hills? It is not pleasing to me to give my law upon high and towering hills. Behold, Mount Sinai is low; and the Word of the Lord has desired to place on it the Divine majesty. Moreover, the Lord dwells for ever in the heaven of heavens." - Targum.
The psalmist is speaking particularly of the mountains of Judea, and those of Gilead; the former were occupied by the Canaanites, and the others by Og, king of Bashan, and Sihon, king of the Amorites, whom Moses defeated.