He was not living a healthy lifestyle to begin with. That happens to a lot of hermits. When you've got no one to impress, why take a bath? Why exercise? Why not eat that entire pie?
Then he jumped into a full night of hard, physical labor followed by the emotional distress of seeing his only hope for the future destroyed. That was too much strain on his aging heart.
Dr. Seuss may not have included this part in his original text, but it is in Kelly Angel's.