maybe they wanted to respect their own fish kind and resume catching human bait on the shores. If you noticed watching the video, there is a lot more sea-life cooperation going on in the ocean than you’d think in the real ocean, so maybe since everyone else was cooperating with each other, the Sharks couldn’t help theirselves to turn a new leaf and join in. The Sharks said “fish are friends, not food”, but they had never said anything about the humans, which brings to me to analyzing that pixar wants sharks to eat humans, not fish (i’m just joking, i don’t think we should get into that much detail). 
So, they haven’t quite gone off the sense of their carnivorous instincts, they’re just acquiring new meat! 