The two most frequent scenarios I have seen in successful Ivy transfer are, one related to a topic/major–not currently offered at their current university and/or a personal situation which is compelling enough to warrant a transfer (and that is assuming you are already qualified). Remember, Princeton takes no transfers, H & Y are at about 1-2% acceptance–and that most transfer seats are tethered to attrition at each respective university–which is very low–about 3% on average.