The FAA extended the required rest period from 8 hours to 10 hours a few years a back. A late arrival the night before could certainly cause a delayed flight at 8:15AM due to the 10 hour required rest period. Not sure why you seem to believe this is so unlikely. You would want to look at the weather on 1/8, not 1/9, to see if the inbound pilots were delayed thus causing a delay on a departing flight the following morning due to required rest.