I reckon it's a consequence of the angle of your forearm relative to your upper arm when tying up the laces. When right handed people tie up their left shoe, the angle at the elbow is more acute than when tying up the right, where it becomes more obtuse. This will effect the leverage, and thus right handed people tie up their left shoes slightly tighter, on average.

If you're left handed, then obviously I'm talking shit, and the complete opposite is true.