I think it might not be any different from their actual reason: a rising china represents a threat to us hegemony and dominance. Remember it’s basically unquestioned amongst a lot of americans that the us is a force for good and that, although the execution or efficacy of us interventions may be called into question, never the righteousness of the us’ dominant position. If china threatens that then there may be debate about the appropriate response, but not about weather they need to be stopped.