Answer
The reason why this occurs is because if there is any apparent wind being pushed towards the direction of the door, there will be an attempt of equalizing pressure and thus it will slam, or close very loudly when you even just push it shut with some effort.
Explanation
This has happened to me a variety of times, I would have the garage open, and it would be a little bit windy, and when I tried to close it, it would slam shut.
Airflow is a significant factor in the pressure appliance.
Your question at the end is correct (Differences in air pressure.)
Apologies for the late response, but I was interested in this as well.