User blog comment:Xilinoc/Ch. 541 - THE BLADE AND ME 2/@comment-10699642-20130613114628/@comment-10630156-20130613122528

my full theory is something like this to go with the whole "working together in a pact" sort of thing and it adds some interesting points as well as answering some lose ends.

I believe that hollow ichigo was being suppressed by "Zangetsu" so he wouldn't take physical form in his inner world in the beginning. But when Ichigo was in peril in his fight will Byakura "Zangetsu" released him from where he sealed him making a pact to help train ichigo and more immediately to KEEP HIM ALIVE. This pact was in force between the two until the time when Hollow Ichigo attacked Uryu and then he was sealed again by an annoyed "Zangetsu" (hence why ichigo couldn't use his hollow mask). Later when Ichigo was trying to get FGT he brought hollow ichigo out again as an ally so that this wouldn't happen but has kept him sealed ever since.

It could explain why Hollow ichigo vanished after each fight in the inner world put it simply "Zangetsu" let him out and resealed him every time he was needed.

It is a crazy theory but does make sense