Entertainers Beyoncé and Jay-Z and billionaire hedge-fund executive Ken Griffin have something in common: They are among a small but growing number of ultraluxury home buyers who are borrowing tens of millions of dollars for home purchases.

The trend bucks the tradition of the ultrawealthy paying cash for their super-pricey homes. Mortgage experts attribute the shift toward so-called “superjumbo loans” over the past couple of years to rising real-estate prices across the country and the historically low interest rate environment,...