I could understand them wanting TSMC to stop buying from Huawei, but why this?

Because all this "national security" is really about trade not national security. Trump declared national security on steel, cars and pretty much any important industry that US didn't lead.

Incorrect. While the Trump administration is more enthusiastic about aggressive intervention in commerce in general, the recent sanctions are the culmination of over a decade of bad behavior by Huawei, recognized by many countries:

https://en.wikipedia.org/wiki/Criticism_of_Huawei

How were the mini-vans from Canada a national security threat?

As far as technology is concerned US is using its corporations(Apple, Cisco, microsoft etc) for espionage as well so there is nothing new here.

I think the assertion is that US has lost core competency at things like steel and cars, so by making it harder to bring in mini-vans from Canada or whatever, they're proposing a magic rebirth of those industries in country. Those are the same competencies we'd need if we wanted to build tanks or battleships or bombers for another assault on Normandy. Which we don't, that was important 80 years ago but not now.

The reality of course is probably more like pandering to steel and auto workers.

> https://en.wikipedia.org/wiki/Criticism_of_Huawei

I see lots of accusations, but very little evidence

Communist China intends to conquer Taiwan. If that happens, all Taiwanese companies will be under the de facto control of the Chinese Communist Party. Selling to Huawei most likely aids Communist China in acheiving it's long-term goal of conquering Taiwan.

So if the owners of a Taiwanese company prefer to control the company rather than cede de facto control to the Chinese Communist Party, this seems entirely reasonable.

If I was the Taiwanese leadership, I'd make it very clear we'd prepositioned enough explosives that in a matter of hours all the top fabs and the like could be turned into junk, denying the PRC these crown jewels.

Just speaking for myself, I've got plans for the inevitable disruptions such a war would create. We'd be throwing away a lot less electronic gear for quite some time.