I thought it was clearly explained, the SO6P had two sons, and when it was time to choose his successor, he choose the younger son. Which made the oldest son hate his younger brother. The oldest sons descendants became the Uchiha, and the younger sons descendants became the senju.well that isnt true there.at least i think it isnt.the way i understood it uchiha and senju werent the actual two brothers but they derived from them...so that could mean uchiha and senju are a little more mixed in blood.only a thought though...it could be well possible that they were brothers