Water had been seeping into the back door during rain storms, for who knows how long. It rotted out the bottom of the back door, but I repaired it by removing the damaged wood, gluing/nailing in new wood and gluing everything back together.
Since then we have added a metal awning above the back door to keep this from happening again. You can see it in this photo with the deck mounted patio umbrella Chelle scored on Hayneedle for twenty bucks.