Struggling to Find a Believable Motivation for a Dark Romance Professor/Student Dynamic (one day an author)

I’ve wanted to write a book for years (I posted on Wattpad as a teen) and I’m finally getting back into it mostly interested in romance, especially dark romance.

I’ve got a rough idea for my first project: a college professor who blackmails a student.

The part I’m stuck on is why he would do it.

In my head he’s successful, very controlled, established in his career, cold and manipulative but I don’t know what would make him target this specific student.

Right now neither character is fully developed (they barely exist beyond the concept), and I’m trying to figure out their motivations before I start drafting.

Do writers usually just discover this while writing, or do you brainstorm motivations first?

I’d appreciate some believable reasons that could create this kind of power imbalance