Because one professor at another ivy has discovery mentioned in high school or college textbook. The 1st semester he did not take any class and focus on research in that professor's group. At the end of semester that group asked him to look for other group. It may be normal because it is two way selection process. It could be brutal to young kids. Hard to change subfield.
Take away from this is to pay some attention to program flexibility. Just like it is easier to change major at private college.
