I saw that coming from teh looks he was giving Ichigo when he showed up.

Ichigo has become a damn nuisance to him; he used the first chance he got to try and put him away for good. I wouldn't call it dishonorable - he's just fed up. Though I don't think he counted on Ichigo being stronger. It's like Renji said, Byakuya takes no notice of anyone else, so Ichigo's little power boost is really gonna take him by surprise.