Adam Clarke on Genesis 21:9

Mocking - What was implied in this mocking is not known. St. Paul, Gal 4:29, calls it persecuting; but it is likely he meant no more than some species of ridicule used by Ishmael on the occasion, and probably with respect to the age of Sarah at Isaac's birth, and her previous barrenness. Jonathan ben Uzziel and the Jerusalem Targum represent Ishmael as performing some idolatrous rite on the occasion, and that this had given the offense to Sarah. Conjectures are as useless as they are endless. Whatever it was, it became the occasion of the expulsion of himself and mother. Several authors are of opinion that the Egyptian bondage of four hundred years, mentioned Gen 15:13, commenced with this persecution of the righteous seed by the son of an Egyptian woman.