Is he doing this on purpose?

***Warning**** this is a really gross story.

J has had a bit of an "issue" lately that involves the following:

a highchair
poop
curious toddler hands and feet
a dog that's not afraid to eat anything

It pretty much goes in that order: We put him in the highchair for lunch or dinner, he's eating, everything is going along just fine. We step out of the room for whatever reason and come back to find poop on his hands & feet, all over the bottom of the highchair seat and on the footrest, and the dog standing there helping to clean it all up!

I don't know if it's the diapers we're using or the way he sits in the highchair, but the poop just tends to shoot straight out his shorts and onto everything. This has happened 4 times now! I guess out of all the meals he eats, that's a pretty small percentage of occurrence, but it's still gross!

And I hate cleaning it up - I'm actually procrastinating doing so right now. I got him bathed and cleaned up, and I sprayed down the highchair with clorox, but I need to go back up and wipe it down and wash the seat cover. Does anyone else's child do this or is is just mine?