Sometimes the entire pod can be navigating perfectly, but they're actually being hunted. There might be a killof owl that is trying to get a group o whales or dolphins into an area. Other times it might just come down to illness. A whale might be too weak to fight the current. These animals are susceptible to disease like we are as humans. Even if you do get proof that a whale died of an illness or got lost, you can't always rule out human involvement. Less food leads to hungrier, weaker whales who might be more pible to disease and then get stranded.