Little Balls of Poop Falling out of Cat

This just recently started. He's still going in his litter box, nothing about his bowel movements has changed. He's neutered and almost 2 years old. We took him in as a stray when he was a kitten so we haven't  clue when it comes to breed, but he is a long hair. It tends to happen if he takes off running or jumps if something scares him. Sometimes he just sits down and when he gets up and walks away there's just a little ball of poop left there. It's not just getting stuck on him a falling off, it's always a pretty decent size ball of poop. It's the consistency of normal poop too. We did just change his food from Friskees to Purina, everything else is the same. Any ideas as to why he's just losing his poop?

Food changes can sometimes cause upset tummies, so soft poops can occur from that, but if he's just leaving normal ones without any effort or anything, I would be a bit concerned he may have some incontinence or a problem with his anal glads:

http://www.peteducation.com/article.cfm?c=1+1322&aid=509

http://www.petplace.com/cats/anal-sac-disease-in-cats/page1.aspx

Sometimes it can be a spine issue (for example, Manx cats who have shorter tails and spines are more prone to problems related to the back end, which includes urinary and fecal incontinence), or a nerve problem, or a muscle problem, sometimes it can be parasites - though maybe less common if the poops haven't changed and are still fairly solid.

The first and best course of action would be to have him seen by a vet so they can narrow down the cause. Best of luck!