Not OP, but there are some common problems, assuming they were a non-founder CTO:

- Instead of leading an eng team, maybe CTO meant "our entire tech team".

- Maybe the founders sucked, or had personal conflicts.

- Maybe the business was broken in ways a technologist can't fix.

- Maybe the business was viable but this individual did not enjoy being a part of it. Could be anything from "there are too many salespeople yelling into phones all around me" to "the technical problems were not the sort I am interested in / good at solving".