It's not unheard of for ogryn to turn to chaos. Rare, sure, but far from unheard of. They are the result of worlds devoted entirely to prisons being left to their own devices during the dark times. Sure, they were eager to join up with the emperor, but their culture before hand was a world ruled entirely by size and strength. That, and as weak-willed as they are, it's also not unheard of for them to become possessed. There are any number of ways to make an ogryn a chaos follower.

Hell, Urgrugg's always happy to instill a willing mind with a warrior of his great, green, cunningly brutal god.