Most researchers believe the phrase originated from a "Deep Web" video title or a corrupted metadata file from the early 2010s. The word "Rotating" suggests a mechanical or looping nature—perhaps a carnival ride or a repetitive animation—while "Molester" and "Train" create a jarring, predatory image that sticks in the human psyche.
It sounds similar to titles used in internet horror subcultures or "urban legend" style storytelling. A Misremembered Title: the rotating molester train exclusive