* Parking Pawl Failure: This is the most probable cause. The parking pawl is a component within the transmission that engages to lock the output shaft, preventing the car from rolling. If it's worn, broken, or not engaging properly, the car will roll. This requires professional repair.
* Transmission Problems: More extensive transmission issues beyond the parking pawl could be at fault. Low transmission fluid, internal damage, or a failing transmission itself could allow the car to roll even in park.
* Parking Brake Issues: While the parking brake (emergency brake) is separate from the transmission's park function, a malfunctioning parking brake could exacerbate the problem if the pawl is already weak. A poorly adjusted or worn parking brake may not hold the car even on a slight incline.
* Incorrect Parking Procedure: While less likely given your description, make absolutely sure you've fully engaged the parking brake *and* shifted completely into Park before exiting the vehicle. A partially engaged park or a forgotten parking brake will allow the car to roll.
What to Do:
* Do NOT drive the car. Driving with a faulty parking pawl or transmission can cause significant further damage.
* Have it towed to a qualified mechanic or transmission specialist. Trying to diagnose and repair this yourself is strongly discouraged; it's a complex mechanical problem requiring specialized tools and knowledge.
* Mention the rolling in park explicitly when you describe the problem to the mechanic.
This is a serious safety hazard. Do not take any chances. Get the car towed immediately.
