What I don’t get is what factors could have caused it to implode now when it didn’t previously?

Material failure after a certain number of stress cycles.

Failure modes for advanced composites are less well understood than for traditional metals as well. The sub's pressure hull was also made out of three disparate materials joined together which adds additional complications. Carbon Fiber in particular is notorious for performing flawlessly until it catastrophically fails in an instant.

I know doing amateur rocketry pressure vessels work until they don't. Motor cases will gladly handle multiple launches and then on the 20th launch, explode. I think it's a matter of the metal fatiguing over time but I'm not sure how you measure the rate or severity.

Fatigue failure:

https://community.sw.siemens.com/s/article/what-is-a-sn-curv...