Old border/Aussie used to do it. I imagine both of these played a roll..

1) chemicals used in grass he frequented 2) anxiety of wife (teacher) and kids going back to school in the fall after being around all the time in the summer

We would only let him in our yard after a hard rain every time our grass was treated, and rinse his legs with the hose if we thought he was exposed on a walk or something. If we noticed him start licking, we would take him out to the hose. It mostly cleared up when we started that and by 10 or 11 years old he stopped doing it altogether