Yes it is sad but fish die from a myriad of causes, old age, disease, poor angler handling etc.
As an angler myself it is one of the most sad sights to see prime specimens dead; but fish do die.
Sometimes the poor handling of Pike of this size by inexperienced anglers is the key culprit although clearly this is not know at this time.
I am sure the EA will look into it and attempt to establish the cause meantime as the PAC bod says - be careful with them.