Sforno on Leviticus 20:9/ספורנו על ויקרא כ׳:ט׳

1כי איש איש אשר יקלל, proof that I, G’d, am so insistent that your sanctity be manifest by your seed being genealogically pure is that the penalty for non observance is the execution of the person who curses his parents. The normal scenario which leads to a son or daughter cursing their parents has to do with legitimacy or otherwise of their offspring. When a son or daughter are the product of unions forbidden under the laws of incest, children of such unions have little reason to practice the commandment to honour their parents as it was from their parents that they learned to ignore Torah legislation. Solomon’s well known instruction שמע בני מוסר אביך ואל תטוש תורת אמך, “heed the moral instruction of your father, and do not ignore the teachings of your mother” (Proverbs 1,8) are most difficult to honour when they know they have seen the light of day only because their parents chose to ignore this very instruction.