colpuck

There was a song can't remember the title, but there was a line..."bet you think this song is about you" which kind of sums up OP's experience.

The issue isn't really with OP's A/C, that was just a downstream effect of what did happen. The issue was with the EWR-LAX flight on 6/13 and probably even before that. The A/C scheduled for that flight developed a fault somewhere, maybe deferrable maybe not. But the end result was about 18hrs before the EWR-LAX ops knew that they had a flight on paper but no metal to operate it. For whatever reason the best course of action was to take N793UA that was scheduled on several SFO-EWR turns and send it to LAX. Okay that solves the problem EWR-LAX, but created one for EWR-SFO-EWR, OP's original flight. UA has N773UA available but it was scheduled for overnight MX at SFO. As OP's flight was originally canceled, I think that was something in the overnight MX package that was non-deferrable and was going to drop dead, I.E. ground the A/C if not fixed. But UA had time, remember we were still a ways out from departure on a weekday flight. The people in charge of A/C scheduling talk to tech ops and probably reg affairs to see if the task that could be extended. Maybe there was even a call to the FAA to get approval. (deferrals can only be extended twice before the FAA has to sign off on them.) Anyways the extension was granted for at least 72 hrs as N773UA just got its overnight last night. The extension allowed UA to build a new flight to get everyone to EWR. Which I am guessing OP was on.