In the film "The Incredible Shrinking Man", what caused the leading character Scott to shrink?

In the 1957 science fiction film "The Incredible Shrinking Man", a strange fog enveloped the leading character Scott (Grant Williams). This fog causes Scott to shrink. The film is directed by Jack Arnold and is based on Richard Matheson's 1956 novel "The Shrinking Man". Besides starring Williams, it stars Randy Stuart as his wife Louise.

The film's storyline focuses on a big event that changes Robert "Scott" Carey. While relaxing on a boat, Scott is enveloped by a fog. Months later, he discovers that he appears to be shrinking. By the time Scott has reached the height of a small boy, his condition becomes known to the public. When he learns there is no cure for his condition, he lashes out at his wife. When Scott shrinks to the point he can fit into a doll house, he has a battle with his family cat. This fight leaves him lost and alone in his basement. He has reached the point where he is smaller than the average insect.

The film was produced by Albert Zugsmith, and he initially wanted Dan O'Herlihy to play the role of Scott Carey. Other cast members were Paul Langton as Charlie Carey, April Kent as Clarice Bruce, Raymond Bailey as Dr. Silver, William Schallert as Dr. Bramson, Diana Darrin as the nurse, Billy Curtis as midget, and Orangey as Butch the cat. Cinematography and editing were done by Ellis W. Carter and Al Joseph, respectively. Universal-International Pictures conducted production and Universal Pictures first distributed the film in February 1957.