Here's my theory...
Remember Isshin's reference to how Juhabach gets his powers back? His "Holy Selection"?
Well, if we're to believe that Zangetsu is in fact Juhabach's essence from 1000 years ago, and part of Ichigo's Quincy powers, then it would make sense that Ichigo's powers were reawakened... Remember Juhabach explained that Kirge's jail caused Ichigo to awaken and realize these powers.
So, my theory is that when Tensa Zangetsu was broken by Haschwald, this was when Juhabach absorbed Ichigo's Quincy powers back.
Basically, Kubo's been trolling us this entire time... Ichigo's a Quincy, and it's only now in this final arc do we get to see his true Shinigami powers...
=.=