From what I remember reading at the time...
the terrorists actually bought blocks of tickets to make sure the planes didn't have too many people on board. If I'm remembering accurately, then suddenly having a rash of no-shows might raise security flags for a given flight.
For all any of us know, they might've chosen those early flight times because of the number of 767s leaving from the Northeast heavily fueled for long flights. They would've wanted all of the hijackings to take place within a brief timespan so there wouldn't be much time for the system to react.