Many, many months ago, Yoruichi came to ask Urahara on how ways to take her skills to the next level. In return, Urahara gave her a new pill he had been working on. Immediately after taking the said pill, Yoruichi's own Reiatsu began surging uncontrollably and began transforming into her thunder beast form, becoming a powerful but simple-minded cat of lightning. The pill, however, also had the effect of greatly increasing her desire to mate. Thus, for the rest of the day, she mated passionately with the closest person next to her, which happened to be Urahara.
The next day, Yoruichi woke up. She had completely forgotten about what she was doing the day before nor could she remember why she came to see Urahara. Although her body felt great, but little did she know that her subconscious mind had been altered by her actions from last night, making her subservient to Urahara without realizing.
Later, Yoruichi would invent a new technique and went on to possess Orihime, thus starting her misschiefs.