Not sure what conclusion you draw from that but it's certainly odd considering he was supposed to be such a master of the strike zone.
Haven't looked closely (or at all - who cares, since he's not a Nats prospect anymore), but a couple theories/wild-ass guesses:
1. He's not as great around the strike zone as you think - A and AA pitchers have less control and so miss the strike zone by a lot. AAA guys miss closer, so he swings more, resulting in better BA and less walks.
2. He's getting fooled into swinging at better breaking stuff - typically AAA pitchers have pretty decent breaking stuff, but have some flaw that keeps them from being MLB - not enough MPH separation between fastball and breaking stuff, fastball too straight, breaking stuff too inconsistent, etc. But anyway, they typically also have some fairly wicked breaking stuff. Could be he's mis-reading that and getting fooled into swinging at stuff that would be balls.
3. He's close to MLB and knows it so is trying to force the issue by hitting his way in - harder to be patient if you think one hot month will get you called up - maybe he's trying to show off power or is being coached to swing more.