James Cameron gave an interview where he claimed the carbon fiber hull concept was tested by a team competing to get down to Challenger Deep at the same time his team was - and the concept failed in testing. He also claimed many within the deep submersible community wrote letters to OceanGate warning them their plan was flawed and would result in catastrophic loss.
Specifically, the failure mode was delamination.
I can't vouch for how credible this all is - but, given how much disregard the OceanGate team had for safety, it doesn't seem too far fetched.