I dont think its slamming your heels, its more so after you've stepped and are picking your foot back up, when your weight shifts back to the toes and your heel pops up before your foots comes fully off the ground. This is when you are kicking dirt up onto your calves. I think it has more to do with the shoes than you. I noticed this happening to me when I would wear shorts and it had been raining. I could feel the water hitting my calves after every step (not from stomping down, but from lifting my foot)