when any of the fish usually die almost every living fish give some nibble to dead fish's fins and tails, nipping might not be the cause of death, i mean to say it is most probably water quality issue may along with fin rot. Regular 30%water changes may help both issues use some salt or malachite along and see. Guppies play chasing each other, watch them a moment and you will know the extent of aggression in chase. Good luck to you and your fish.