It looks like this particular die-off was more severe than it otherwise would have been, due to unusually warm ocean temperatures and the resulting higher calor

It looks like this particular die-off was more severe than it otherwise would have been, due to unusually warm ocean temperatures and the resulting higher caloric need for the crabs. So the cycle is natural, but the extreme depth of this particular "trough" in the cycle seems like an abnormal event. (cross-posted from another comment, but seems relevant here too)