Hmmmm, just throwing it out there, but what if, just WHAT IF, Aya had read enough manga to know that at this point she's gonna have to chase Mitsuki all over God's creation to make any progress happen, SO, she knocks her off the stairs to slow her down enough to actually make some sort of progress. Abusive and evil? Yes, but also effective. Also gives her an excuse to hang out at her house even more and faun over and spoil her crush.