Pooping in house after a Long Walk

I have a 7 month old Male JRT I adopted from the local Humane Society. He is well mannered and has a good disposition. I crated him for the first week at night. Now at night he is confined to the kitchen. I work at home and restrict his run of the townhouse.( not allowed upstairs) He does not have accidents at night or when I leave him alone. When I take him for long walks and he urinates and defecates with regularity. However, about half the time he will come back in the house and poop or pee very quickly. Just as I release the leash and he darts off somewhere very fast and does it. Usually it is urinate. This morning he pooped two times very quickly. This was after a 20 minute vigourous walk where he did all his business.