We are talking about the same thing in my Weed Science class.  My professor seems to think that waterhemp has a high ability to mutate naturally and the already present gene or ability to resist glyphosate was already present. The reason it is showing up is due entirely to increased selection pressure.  He went into talking about how waterhemp and species like waterhemp (palmer, pigweed) only produce seeds through cross pollination and cannot pollinate themselves.  This results in more genetic variation.  That’s what I gathered through the lectures anyway.  Hope that is understandable.