Rubin was asked to step away from running Android because he was not collaborating well with other teams within Google and with external Android partners. He is tremendously smart, accomplished, and successful, but he can be difficult to work with and was antagonizing external partners like Samsung (which badly want to build their own OS but don't have the expertise to do so successfully). The robotics group was a consolation prize given to him after taking Android away from him. As to his departure from the robotics group, my guess is that he and Page don't see eye to eye on how to run it, and he was asked to step away.