Gregory's Server
A hypothesis not based on anthropogenic beliefs in magic based death cults would be that many species go off to die when they sense their time is up.
Whales do this by beaching themselves, which is why many beach again after being rescued and put back in the ocean.(A lone whale beaching should be left alone to die in peace, imo. Not traumatized and terrorized by hordes of ppl interfering with it's last moments on earth.)
Mass beachings are probably caused by human activity in the ocean; blasting, sonar, swimming thru pockets of extreme pollution - which sickens or harms most of them in the pod to a fatal degree.
Human activity in the ocean can also disorient whales, which depend heavily on their sonar abilities.