All my dogs including Jedda have done that, too. Even though Jedda's on raw now and has compact little poos at home, she'll still have a softy about 10 minutes into a hike. I think what wouldn't normally come out for a while, and would have more water absorbed by the intestines, gets stimulated to be evacuated by the exercise. In my opinion, no worries! I don't think adding fiber would necessarily change anything about exercise poo!