Board Thread:General Discussion/@comment-4161952-20130331221843/@comment-7313057-20130402163658

Aizen probably foreseen Tosen's betrayal since Tosen was blind and unaffected by Kyoka Suigetsu. And he knew since he gives Tosen hollow power that he could attack anytime, so he made sure to give Tosen Resurrección form sight in the event they face off against each other.

While Tosen was on his death bed he had a flashback of his unnamed female who was killed by her husband. I believe the unnamed husband was Aizen since Aizen was also part of the flashback.