Albert Barnes Commentary Isaiah 64:3

"When thou didst terrible things which we looked not for, thou camest down, the mountains quaked at thy presence." — Isaiah 64:3 (ASV)

When you did terrible things – in delivering the people from Egypt, and in conducting them to the promised land.

Which we did not look for – which we had never before witnessed, and which we had no right to expect.

You came down – as on Mount Sinai.

The mountains flowed down – (See the notes above). The reference is to the manifestations of smoke and fire when Yahweh descended on Mount Sinai .