I have a question about Lee Sin's ultimate

I've recently played a few Lee Sin games where I ulted, and something I wasn't expecting happened. This most recent game, Akali was diving me under my tower, she was in range and the tower was focusing her (she had tower agro). I ulted her, and kicked her closer to the tower, assuming the tower would continue to focus her, but the tower changed focus to a minion. I don't understand why that happened, she was in range of tower and I kicked her closer to tower, so she never left agro circle, but the agro shifted. I'm not sure if this is a bug or I just never knew about it, but it's happened in multiple games and I was just wondering if I should not ult an enemy when they have agro or if this is just a bug.