Taylor Swift has more than 119 million followers on Instagram. Scooter Braun, who manages Justin Bieber, Ariana Grande and Demi Lovato, has just over 3 million.

That number appears to be dropping.

It all started on Sunday morning, when Mr. Braun’s company, Ithaca Holdings, announced that it had acquired the Big Machine Label Group, which Ms. Swift left in 2018 and with which she released her first six albums, for $300 million, according to The Wall Street Journal. In so doing, Mr. Braun became the owner of the masters to Ms. Swift’s catalog — everything leading up to the singles from her forthcoming album, “Lover.”

Ms. Swift responded to the news on Tumblr, where she has in recent years cultivated a supportive following of Swifties. “This is my worst case scenario,” she wrote of the Big Machine sale. “This is what happens when you sign a deal at fifteen to someone for whom the term ‘loyalty’ is clearly just a contractual concept. And when that man says ‘Music has value’, he means its value is beholden to men who had no part in creating it.”

[Read more on the label’s sale and what it means for Taylor Swift.]

By Sunday evening, the hashtag #WeStandWithTaylor was trending on Twitter, and fans showed their solidarity by unfollowing Mr. Braun on Instagram. Ms. Grande posted an Instagram Story congratulating Mr. Braun on the deal, then deleted it, which some have read as covert support for Ms. Swift.