Two things. First, military security is entirely dependent on economic security. The productive capacity of a country's economy places a hard upper limit on its ability to produce materiel and thus effectively wage war. For obvious reasons trade can't be relied on when at war with the former trading partner.

Second, this administration has been promising a trade war with China since before the election. It's not as if this is some kind of surprise. "CHYNA!" was a major campaign issue.

For background: in the Nixon era, the USA decided that it would start a Marshall plan style trade deficit funded "investment" in China, with the belief that increasing the economic prosperity of the PRC would lead it to greater openness and generally becoming a tame satrapy of the US empire, much as Germany and Japan did. Now that it's clear that plan was a complete failure and the Chinese played the Americans for fools, some Americans, most notable among them the President, are looking to reset to a less inequitable trade relationship. Pretty much all major economic and political news from the pacific region is a consequence of this process. The Hong Kong riots? Turns out the CIA is involved[1]. North Korean missile launches? Yeah, how much real autonomy does the DPRK have when China decides to call shots?

Edit: [1] https://www.reuters.com/video/2019/08/08/us-calls-china-thug... (Obviously this is a pro-US outlet and frames it as innocuously as possible, but anyone who knows anything about tradecraft knows that "diplomat" in this context is a polite word for operative).