I came over to the dark side from software development. Was tired of implementing other people’s bad ideas and convinced product leadership at my company that I could be an idea guy myself (along with all the other footwork and juggling you need to do as a PM). So it was an internal transfer. I think the good PMs have strong hands-on “actual development” backgrounds. I realize this is a self-serving opinion, too.
2) Internal transfers - Software Engineers switch to PM
3) Management/Technology Consultant -> PM: Folks switch from consulting (where they are implementing products designed by vendors) to becoming PMs in the vendor companies.
Most PMs I know are former software engineers (or if hired as PMs out of school, studied computer science) with good people skills and high level knowledge of what they help coordinate. I would hate to work with an MBA-type PM who couldn’t appreciate technical problems and solutions
Edit: this does raise the question, though: where do product managers come from? What does the career path of a PM look like, including what happens before they become PMs?