Charles Ellicott Commentary Exodus 8:13-14

"And Jehovah did according to the word of Moses; and the frogs died out of the houses, out of the courts, and out of the fields. And they gathered them together in heaps; and the land stank." — Exodus 8:13-14 (ASV)

The frogs died. —God, who knew the heart of Pharaoh, and its insincerity, or at any rate its fickleness, took the plague of frogs away in a manner that made its removal almost as bad as its continuance. The frogs did not return into the river; neither were they devoured by flights of cranes or ibises. They simply died—died where they were—in thousands and tens of thousands, so that they had to be gathered upon heaps. And the land stank.

In the great plague of frogs mentioned by Eustathius (see the comment on Exodus 8:1-4), it was the stench of the frogs after they were dead which caused the people to quit their country.