1) Straight from Business Schools - Folks who go into technology tend to become PMs after their MBAs
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.
The internal transfers is much broader than just SWE to PM, as well. UX designers, technical writers, QA testers, etc. Anyone working on a dev team might start picking up PM responsibilities and make that transition at some point.
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.