The morning after he uploaded his new movie “Froning: The Fittest Man in History” to the iTunes store, in early October 2015, the director Heber Cannon received a phone call from somebody at iTunes, who sounded bewildered.

“Froning,” a documentary about the four-time CrossFit Games champion Rich Froning Jr. that Cannon made as an employee of CrossFit Inc.’s media team, had shot overnight to the top of the U.S. iTunes sales chart for independent movies, and was sitting in third overall, directly below “Jurassic World” and the Melissa McCarthy comedy “Spy.” According to Cannon, the iTunes rep wanted to know how a film they’d never heard of had wound up beside blockbusters.

“They called us and said, ‘Who are you guys? How did you do this? What’s going on?’” Cannon said in a recent video interview. “We didn’t even have distribution. We had just done it ourselves. But when we dropped the film, it was like wildfire.”

Cannon and his creative partner…