Dumbfounding Door

A few minutes ago I went out of the back door of the house where I live, to throw some de-icing particles on part of the back lot. I closed the door behind me as I went out of the house. When I approached the door to return to the house, at the moment I was about two feet away from the door  the door mysteriously opened about one foot. There was no noticeable wind gust outside; I could not feel any wind;  and the listed wind speed for my city at this hour is only 13 miles per hour.  There was no one in the house when the door opened. I've never seen the door behave this way before in the years I've lived here.  For another account of a door mysteriously opening, read here.