I think because China has very high growth momentum that surpasses american living standards soon, and not long before it will surpass american security standards too. China purchasing power is probably more comfortable than most west countries, with extensive housing and high speed rail and electric cars etc. When a country becomes rich, inevitably other countries ask for their help. That's why china's growth must be curbed, fast.

Because China started threatening war. It's not really sudden. It's from Xi coming to power and saying he'll take Taiwan.

Because the U.S. wants global hegemony and time is against the U.S. so there is a rush to crush China sooner rather than later.