I had a great example of this happen to me once. Red-eye's to BOS and NYC (not JFK, since wasn't p.s. equip.) were scheduled to leave at the end of the pier in SFO at gates 89 and 90. Well my original aircraft bound for BOS got rejected by the pilot due to a faulty smoke detector in a lavatory (at least that was the detail I thought I overheard as the reason.) This announcement was made by our GA and then the NYC GA who had waited for that announcement to finish called initial boarding for the NYC flight. Our GA then announced a "Gate Change" to the NYC gate. Followed immediately by their GA announcing they also had a gate change to the BOS gate. Now these gates are adjacent and at the end of the pier, so the seats are shared, so no one had to move except to look around in confusion.

Two minutes later we ( the folks bound to BOS) began boarding the plane we had just stolen from the New York folks, and as I was boarding I heard them make an announcement that the NYC crew had also rejected the plane and that they would need to wait until 3:40am!! for a replacement plane to arrive. Needless to say I was as I nestled into my seat, and the NYC bound passengers were irate that "the computer" had stolen their plane and given it to the Boston folks. They were just seconds away from having boarded that plane and making it out on time.

So I do think pilot's exercising their discretion in rejecting aircraft that haven't yet technically been deemed unable to fly (based on MEL or other standards) can be a likely scenario for such inexplicable swaps.

Interesting example. How many radars are their onboard standard jets? I always assumed one (primarily intended for WX and terrain) and maybe two (TCAS, transponders). I would never have guessed a jetliner could fly without a working WX radar? Can they fly like that at night, still do ILS landings?

I am not even a flight simulator pilot, just a curious electrical engineer.