I don't claim to know what actually happened, but one possible cause is that the HSM (required for issuing and renewing intermediate certificates, mentioned in

I don't claim to know what actually happened, but one possible cause is that the HSM (required for issuing and renewing intermediate certificates, mentioned in the post) might require a right person and right schedule to operate. The use of an HSM means that you don't normally use it to issue certificates and you should only touch it a few times a year. As a result there may have been only a few people with an effective knowledge of the HSM; when they can't be scheduled for some various reasons, well, it may be forgotten. While it of course sucks, something like this happens anywhere anytime.