Sforno on Exodus 8:28/ספורנו על שמות ח׳:כ״ח

1גם בפעם הזאת, as he had done after the plague of the crocodiles had been removed. Seeing that the wild beasts had only retreated to their lairs and had not died, so that they still represented a potential threat, Pharaoh toughened his resolve and refused to send off the people.