It's thought that schools reject anyone they think has a low chance of matriculating, since rankings are influenced by a school's yield (i.e. the amount of people they have to accept to fill the seats they want). A high yield, meaning less acceptances sent out, is what each school wants. It's thought that schools, rather unfortunately, just reject applicants who's stats are high enough that the school doesn't actually think that applicant will go there. This includes more of your mid and lower tier schools that have worse yields than higher ranked schools. I have no idea if this really happens, but I think that it's been confirmed by adcoms on SDN before.