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I believe that Aizen planed this from the start in order to get the Hougyoku. He must have known that Shunsui and Ukitake would try to free Rukia and that Yama-yii would fight against his two students. Only that way Aizen could be sure not to have to fight against the 10 remaining Captains at the same time during Rukia's execution to get the Hougyoku from Rukia.
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