Garth Brooks frequently collaborates with his wife Trisha Yearwood, but many fans might not know that he co-wrote one of his biggest hits with his first wife, Sandy Mahl. Brooks collaborated with Mahl on “That Summer,” but only after the original idea that he had started with songwriter Pat Alger didn’t quite pan out. In a 1996 television special, Brooks recalled that the original story was about a married woman whose husband is ignoring her at a party, so she goes off and has an affair with a single guy. Brooks’ producer, Allen Reynolds, told him he couldn’t root for the characters in the son…