Families that are high, fall down and become low owing to the absence of sacrifices, impure marriages, abandonment of the Vedas, and insults offered to Brahmanas. High families fall off and become low owing to their members disregarding or speaking ill of Brahmanas, or to the misappropriation, O Bharata, of what had been deposited with them by others.

In plain words: Good families sink low when they give up worship and the sacred texts, make improper marriages, and insult or speak badly of Brahmanas (priests and teachers). They also fall when they steal what others left in their care.