I was on a flight Monday that was already running late due to problems at ORD. We closed the door. Then the pilot came on and said a problem they fixed earlier came back. We opened the door. A few minutes later (I think before maintenance came onboard), someone from the ground (not sure if it was a GA) said pilot timed out and everyone off the plane while they looked for replacement.

It was about 7:15 PM, I think. Soon after we deplaned, the flight was changed to the next morning at 6:30. I got myself moved to the redeye.

On the redeye, there were many fellow passengers who also moved instead of taking the re-timed flight. One woman told me what happened was that the pilots did not really time out, they declared "fatigue". She said that there is the mandatory FAA timeout rule but pilots can also declare fatigue.

How do the rules work? Any idea what might have happened?

Thanks.