How did Leopold I become heir to the Habsburg lands after Ferdinand IV died?

Leopold I became the heir to the Habsburg lands following the unexpected death of his elder brother, Ferdinand IV, on July 9, 1654.12

Prior to this event, Leopold had been destined for a career in the church, while Ferdinand IV—the eldest son of Emperor Ferdinand III—was specifically groomed for the imperial throne and had already been crowned king of the Romans. Upon Ferdinand IV's premature death at age 21, Leopold suddenly became the heir apparent to the Austrian Habsburg lands. He subsequently received homage from the Lower Austrian estates in 1655 and was crowned king of Hungary and Bohemia before eventually succeeding his father as Holy Roman emperor in 1658.12