Board Thread:General Discussion/@comment-26475399-20150621155411/@comment-5398287-20150622063347

He was probably waiting to be freed because he couldn't leave Muken without the door being opened; as soon as he realized the key to said door was within Shunsui's heart and would thus be destroyed if he died, Aizen deduced that this was a countermeasure against him just killing Shunsui and leaving when the latter came calling.

Apparently, the aliens from Signs aren't the only ones to have trouble with doors.