I just ran a test roll of film through my recently acquired GA645Zi. When the negatives came back from the lab they showed significant light leakage. The lab suggested that during loading I might not have wound the film up tightly enough. They also suggested that when I unloaded the film I may not have wound it tightly enough either. The very first frame did not have any light leakage, but all subsequent ones did. As one can see from the sample image below one can even see some darker circles which almost look like sprocket holes. However, to the best of my knowledge MF film doesn't have sprocket holes.

Other than what the lab suggests could there be other causes for the light leakage?