He's surfing the web one night, bored. He stumbles across a cam site, and finds himself scrolling through, only half interested. He finds one girl that looks good and watches for a bit. Then the girl angles the camera up to her face and it's one of his students (or his friend's daughter, or a therapy patient, or any number of things). She's underage and he can use that to his advantage. Looking for him and looking to involve a lot of different kinks (exhibitionism, blackmail, dub-con, sex for money, training, sharing, etc).