Yes, it does provoke (unless it specifically says otherwise,) but keep in mind that combat maneuvers in general only provoke from the target of the maneuver, not from every creature that threatens.
The "free" means "without additional action cost", not "free from potentially provoking an AoO".
What is giving you the free Bull Rush attempt ?
If it comes from Shield Slam, that specifically says the free bull rush doesn't provoke.
If it comes from the Piledriver(Ex) that a Foehammer gets, that also specifically says that it doesn't provoke.
What specific case are you looking at ?