Either because they had been abused to idolatry and superstition at Shechem, or elsewhere, and therefore were to be destroyed according to God's command, now signified to Jacob, and afterwards delivered to his posterity, Deuteronomy 7:5, Deuteronomy 12:3; or for fear they should be so abused. For the Scripture seems to insinuate, and other writers expressly affirm, that divers heathen people did wear earrings for the honour of their idols, and with the representations or ensigns of their idols engraven upon them. See Judges 8:24. After he had melted or broken them, (which seems probable from parallel instances, as Exodus 32:20 2 Kings 18:4), Jacob hid them under a certain oak, though not known to his family which it was. He chose that place, either as most proper to put monuments of idolatry under those trees which were so much and so generally abused to idolatry, as oaks especially were, Isaiah 1:29; or as the safest place, where they were likely to remain longest hid, because the heathen had a veneration for oaks, and therefore would not cut them down, nor dig them up, nor do any thing which had a tendency that way.
Related Commentaries of Genesis 35:4
Genesis 35:4
4 And they gave unto Jacob all the strange gods which were in their hand, and all their earrings which were in their ears; and Jacob hid them under the oak which was by Shechem.