It's unclear the cause of this, one might infer it was US pressure to adhere to the agreement after finding non-compliance, alternatively it could be internally originated investigation.
They were not. Not even "kind of". For them to be fake they'd need to be made out as something they are not[1]. The companies who acquired those certificates just weren't aware that certificates from that particular branch weren't permissible under SK rules - likely not even authorities in SK were aware until they had a closer look.
As an analogy, a driver's license from one country may not allow you to drive a car in another, but that doesn't mean it's a fake license.
[1] Edit: Some outlets claim there were fabricated test records, if that is true and not just journalists writing whatever, then they would be indeed fake.
Hmm, what makes you think that? I didn't see any indication in the article that the Chinese branch was not testing to the standards of US certification.
IMO this is a somewhat grey area. Not familiar with regulations around testing labs, but om the surface looks like an innocent cost optimization that someone decided to put some pressure on.