I believe you just made a mistake, and forgot to fill your food pen for a while. It looks like your pets died as the game intended; they starved to death. I checked all the pets in your graveyard, and they all have zero hunger.
When a pet glitch dies to the grave, it will still have hunger points. Only pets that will glitch die are pets from V1 of Rescreatu that have been sitting in a showroom and have never been out for reset since V2 came to be. We're in V3 now, so that means the only pets that do this are very old. Also, they all have a birthdate that matches the launch date of V2.