Need Explaination

Picture taken from my room window. Using 30sec exposure, f5.6, iso 100, Canon 30D, EFS 55-250mm zoom lens and tripod. Somehow the picture is like slipping towards the bottom left corner. Taken a few more same picture but change the tripod position (in case one leg was slipping slowly) get same result. Earth moving very fast? Or the light is bouncing of the moisture in the air?