My take is that there was a serious "threat" to U.S and occupation governance through political alliances or something behind the scenes that required a purge.

My sense of this comes from the arrest warrants that were immediately dropped on opposition and even the one person from Yoon's own party. U.S is sitting back and casually letting this happen as well; meaning that it definitely isn't something concerning the Empire. I doubt it's a test run or to manufacture consent for something other than stoking more paranoia in South Korea.