Since my ds has been very young, and had MONSTER blowout poops, it has only ever happened if he's ready to go up to the next size. We use a combination of Pampers Cruisers, and Pampers Baby Dry (I like the cruisers for poops especially, but the baby dry are just as good for pee's I think!), and just last week his #2 came up the back of his size 3 cruiser. We're trying to finish up the box, so I know he's ready for size 4's.
If you don't think size is the problem, I agree with the pp's--maybe try re-adjusting the elastic around the legs after a diaper change?
Whiskers