Moses continues reminding the people of Israel what happened in the past.
The word “behold” here shows that Moses was surprised by what he saw.
The Israelites of the earlier generation had asked Aaron to make a metal calf so they could worship it. The full meaning of this statement can be made explicit. (See: rc://en/ta/man/translate/figs-explicit)
Moses speaks as if obeying God’s commands were walking along a path. See how you translated these words in Deuteronomy 9:12. AT: “You had quickly disobeyed what Yahweh had commanded you”