If I remember correctly, the problem was that O-rings were designed to not erode at particular humidities and temperatures, which were subtly but critically different from the launch conditions.

This was actually observed by engineers at the time, who tried (unsuccessfully) to raise the observation to their superiors. The launch went on anyway, and the rest is history.

This has more details, along with my cousin comment: https://people.rit.edu/wlrgsh/FINRobison.pdf