Following the revelation that the last year of the manga was all a part of Aizen's plan to sacrifice all the weak members of his army in order to trap people in Heuco Mundo, Aizen and Yama's forces are finally entering the real battle. With his 3 captains and 3 espada vs. 6 captains, it looks like a pretty even matchup.
Except it's not, because Urahara and the vizard are there. Which means it's actually 6 on 16 and Aizen's walking into a massacre.
So has Aizen got completely insane, or is Urahara somehow being kept out of the battle, or what?