Why is Taylor Swift on the 'Fifty Shades Darker' soundtrack? Here are five theories.

Within hours, the pop-dance track was the No. 1 worldwide trending topic on Twitter and No. 1 on iTunes. ("Well that escalated quickly," Swift wrote on Instagram, as if she didn't know that would happen.)

As Swift has become one of the most powerful celebrities in the world, she's famously methodical about her multi-million dollar brand, and meticulously chooses her collaborators and projects. So why did the calculatedly prim star decide to contribute to a movie based on a best-selling erotica series that delves into the finer points of sadomasochism? And with Zayn, of all people? We have a few theories.

1) The "Fifty Shades" soundtracks are legit.

Although the books and movies are understandably ridiculed, the soundtrack from the first movie is impressive. Fueled with tracks from Beyonce, Sia, Ellie Goulding and The Weeknd, it was a huge success in 2015 and sold about 2.2 million copies. What better way for Swift to continue to conquer the entire global music industry than get in on a hit franchise?

2) She wants an EGOT.

Swift has a bunch of Grammys. And an Emmy (really.) Now all she needs is a Tony and an Oscar. Hey, The Weeknd was nominated for best original song at the Academy Awards last year for his "Fifty Shades" soundtrack contribution, "Earned It," so. . . it could happen.

3) She's doing a favor for Gigi Hadid.

The Victoria's Secret supermodel is one of Swift's closest friends, and we're embarrassed for you if you don't know Hadid is dating Zayn. Anyway, Zayn split from One Direction last year and released his solo debut project in March. While it debuted at No. 1, the buzz has quieted, especially since his smash single "Pillowtalk." What better way to raise his profile than collaborate with T-Swift?

4) She's taking a break between albums while still raking in $$$.

Swift recently made headlines as the highest-paid musician of 2016, earning $170 million . . . and she hasn't released an album since 2014. It doesn't matter – she can take time off from touring and still bring in the dough, thanks to writing for other acts. She wrote her ex-boyfriend Calvin Harris's hit "This is What You Came For" as well as Little Big Town's "Better Man," which is performing solidly on the country charts.

5) She wanted to give her viewing public a present for the holidays.

Well that was nice of her! And no, not the actual song. We mean the gift of allowing us to speculate on the mysterious mechanics behind it. What do Swift and Zayn talk about behind closed doors: His ex-bandmate and her ex-boyfriend, Harry Styles? The time Zayn made fun of Swift on Twitter and Calvin Harris jumped in to defend her? Did Zayn have to apologize? The possibilities are endless.

Author: Emily Yahr covers pop culture and entertainment for the Post.
@emilyyahr
The Washington Post