XC90 rolls even though gear is inserted

Hello XC-90 municipality, I park my XC90 backwards on my yard. To turn off I always insert the first gear and switch off the engine. The parking brake is not operated. The car is standing with very slight gradient. Yesterday I noticed how my XC90 rolled down almost a meter. I could even push it from behind. The whole thing happened a few months ago. What’s going on with my XC90 ?