Y'all need to realize that Top 20 to one employer is different than Top 20 to another employer. For instance, in San Francisco Hastings is probably considered "Top 20."
Basically, Top 20 really has nothing to do with USNEWS, either. These firms have a LONG history hiring students from schools. They will base their determination of the quality of a law school by the quality of lawyer they hire. If a Boston firm has always favored BC grads over BU grads b/c of the type of lawyer they get, that will not change unless their success with BC grads declines and their success with BU grads increases. They really don't give a damn that BC is now ranked below BU in in magazine.
Businesses/firms keep files and statitics on where every hire comes from. If Emory drops another 10 spots next year, yet the firms that recruit there have no drop off in quality hires, those firms won't all of a sudden stop recruiting at Emory.
As for new firms coming to a campus? I would guess this has more to do with lateral hires or partners coming to a firm with a background in recruiting at the new school or are alums from that school.