וַתֹּאכַלְנָה הַפָּרוֹת הָרַקּוֹת וְהָרָעוֹת אֵת שֶׁבַע הַפָּרוֹת הָרִאשֹׁנוֹת הַבְּרִיאֹת
And the lean and sickly cows devoured the first seven healthy cows.
Rabbi Joseph B. Soloveitchik points out that cows are normally herbivorous; they’re not carnivorous and they’re certainly not cannibalistic! Similarly, during the years of plenty, the Egyptians would treat one another well and not envy one another’s prosperity. In the years of famine, however, they would turn on one another and metaphorically “eat one another alive.” Joseph saw that the way to avoid this was by stockpiling food for the coming crisis. (Cited in the Mesoras HaRav Chumash.)