“Pitch Perfect 2” stopped the show at the box office this weekend, grossing an estimated $70.3 million in first place. That debut is about $5 million more than the entire domestic gross of the original film, and more than double the sequel’s $29 million budget. Heading into the weekend, analysts expected the sequel to premiere in the $40 million range. But not much about the “Pitch Perfect” franchise has made sense by traditional Hollywood rules. The original film, an acerbic comedy set in the low-stakes world of collegiate a capella singing, opened in September 2012—hardly a time of year known...