“Trump tried to orchestrate a sale of TikTok to American companies in 2020 …. an effort that was struck down by federal courts. He ​c​hanged his stance on TT last March after he met Jeff Yass, who owns a large share of ByteDance, yet Trump said they did not discuss TT.”
/Paraphrased from: https://www.nytimes.com/article/what-we-know-tiktok.html
So Trump was either bought out, or saw how he could use his own opinion to sway his diehard voters in his direction.