Board Thread:General Discussion/@comment-4115054-20140903063305/@comment-10301320-20150416081633

Mugen Galath wrote: "You have lost, Kurosaki Ichigo. Behold! My Zanpakutou is disappearing. You of all people must understand what this means. The Hōgyoku has determined that I no longer require a Zanpakutou! I will be like you, who became one with his Zanpakutou and his powers! No, I will eclipse you, who have now lost those powers, and conquer heights far beyond your imagination! You're finished, Kurosaki Ichigo!" - Sōsuke Aizen, before Kisuke Urahara's sealing Kidō activates

I always took this as Aizen being arrogant and believing he had done the same thing as Ichigo so that he could feel somewhat superior because at that point in time everything was pretty much falling apart from him and he was being humiliated by Ichigo. There's nothing to say either way what happened, but to me I think that's when the Hogyoku started to reject him and his Zanpakuto disappeared due to Hogyoku-rejection related reasons.