The Lord was about to intervene in the affairs of His people. He is not only transcendent above all but immanently involved in the world, one of the most basic revelations in Old Testament theology. When He came, all the earth would melt, split, and quake before His awesome power (cf. Judges 5:4-5). Since He could affect the physical creation so drastically, His people needed to fear Him. Treading on the high places of the land, where the Israelites worshipped in idolatry (cf. 2 Chronicles 33:17), probably also implies that He would crush pagan worship. [1][source]
"If men would tremble before God, instead of before each other, they would have nothing to fear." [2][source]