I would advise you to consider the weight of the backpack, the angle of his spine and the thickness of his legs. The backpack looks like it is carrying items, which means his center of balance has been shifted back like this:
Note how his shoulders and upper back are thrown forward to help adjust his center of weight.
This might be a fairly slight motion except that your legs are so thin that they exaggerate how little the upper body is reacting to the weight. Because the body isn't hunkering down the way you'd expect under a heavy weight it appears as if the figure is somehow leading with its feet and having the upper body float slightly behind.
