Quote from xdude101
I'm not sure Isshin gets exiled, so much as he loses his powers through the final getsuga. This has been aluded to many times since the revelation that he was a shinigami, most notably when he sneaks into Ishida's training center and Ryuken says something along the lines of "Have your powers re ...
Well we know now that Ichigo got his name from his mother, probably so no one from SS (Urahara or random shinigami passing through) would recognize his name since it is a rather famous surname thus protecting his family from SS interference.
Now that you mention it the final getsuga seems like the likely explanation, but then it would contradict what was said when Ichigo used it since it was supposed to take his powers away forever, but Isshin is regaining his power and that means the Fullbringer arc was useless because it was supposed to be about getting powers back that could not be brought back.
Alternatively I would suggest that Isshin may have lost some of his power from being in the human world too long in a false body and that comment is not about him regaining all his powers but restoring his old level of power.