terrapin died. why?

by Dave
(Cambridge UK)

my 5 year old terrapin has died, he had not woken from winter hibernation in our pond, i check on him regularly to ensure all is well and as in previous years he seemed fine. this morning he was floating around the pond so i obviously thought he had awaken so went to feed him, only to find he was dead.

he has swelling in front of both his rear legs and from his anus were hanging what looked like his entrails, almost as if his digestive system had expanded and blown out of his rear.
any idea what may have caused this?