Screaming like a stuck mynock.

Fett is good at a lot of things, but he has so few opponents that he regularly fights (ok, he's probably had two in his lifetime) that he's a bit rusty on getting "house trained" to face their particular style. So when Atton shifts styles, it throws him pretty badly, and though he recovers...it's only in time to try and drag Atton down into a heap with him rather than to avoid the attack entirely.