Klein explains the matter well:
The ephod is a snare, a lure to worship a tangible in which the people have invested value and form. This ephod introduces a motif which gains in significance in the remaining narratives: Israel's increasing secession from ethical Yahweh worship to worship of humanity's self-determined values, its own creations. This motif represents a shift from
merelyworshiping the local gods with which the Israelites come into contact. Israel is no longer the innocent of the desert who is easily seduced into apostasy. Grown more confident, more worldly, more sophisticated, Israel creates its own trap, its own lure from Yahweh.1
27 And Gideon made an ephod of it and put it in his city, in Ophrah. And all Israel whored after it there, and it became a snare to Gideon and to his family.