Then the earth shook and trembled; or, quailed and quaked (Kay, who thus expresses the assonance of the Hebrew vat-tig'ash vat-tir' ash). The psalmist must not be understood literally. He does not mean that the deliverance came by earthquake, storm, and thunder, but describes the discomfiture and dismay of his opponents by a series of highly poetical images.
In these he, no doubt, follows nature closely, and probably describes what he had seen, heard, and felt. The foundations also of the hills moved and were shaken. In violent earthquakes, the earth seems to rock to its foundations; mountain ranges are sometimes actually elevated to a height of several feet; rocks topple down; and occasionally there are earth-slips of enormous dimensions.
Because he was wroth. God's anger against the psalmist's enemies produced the entire disturbance which he is describing.