Hopefully I'm hyperbolic. I'm not rooting for the collapse of the country I live in, and the country of people who I grew up with and raised me. I just find the truths I trust lie more in laws of nature and laws of power, rather than higher-level societal constructs.
The West Coast is abundant in natural resources, provides good natural port access to the Pacific and Pacific nations, and provides a direct conduit to Alaska and its oil fields. The prospects of invasion depend on a cost benefit analysis.
Maybe cross-border drug traffic increases exponentially without DEA and Border Patrol, and you want to secure a region bent on escalating retaliation. Might also be an opportunity if half the population is too coked out to put up a fight. This was the First Opium War in China and the Century of Humiliation.
Maybe too many Americans are fleeing desperate circumstances into Mexico, and Mexico wants to stop refugee outflows while playing the hero bit. This was Vietnam + Cambodia during the Khmer Rouge.
Maybe global warming causes a chain of collapse towards underfunded and overextended governments in South America, and Mexico is itself invaded from the south and is running north. The collapse of Venezuela is a rehearsal of sorts.
The Federalist Papers contain examples out the wazoo of a "country" turning on itself, as warnings against disunion. Highly encourage reading them.
Quite frankly, it doesn't need to make sense. It needs to be possible.