Given all the very partisan flip-flopping on TikTok, I'm guessing there's not much to the "full and classified reasoning" and the real reason is more like "damn kids don't read my twitface posts, instead they call me names on a Chinese app, China CHina CHIHIANANANADASARtGHHGLE" and "My opponent has a lot of traction on TikTok, let's shut it down".
One way to think about social media is like a weapon. For the history of social media the US consumer was using a weapon that the US government had a way of exercising control over because Facebook, IG etc all existed as product of America. Tiktok was an outside challenger and threatened American power. Hence they took action to see it destroyed.