House Trained Dog Pooping Inside?

My 6 yr old hound mix has started pooping in a line both inside and out, but even more troubling is that he is pooping at random in house in no particular order. He is very perky and has a dog door. But even on walks he has to poop a bit then the remainder falls out while walking. He had his anal glands expelled and a lower back X-ray with no abnormalities. But he seems unable to get to dog door any more in time to poop. Ideas welcome. He does not struggle to squat, either.