It wasn't so much that the aircraft you deplaned from had to get back to BDL, but much more likely that the aircraft you were switched to needed to get to MCO, which is a maintenance facility for us, and where much overnight work is accomplished.

As others have suggested, the exact reason(s) the aircraft swap could have been needed are numerous and varied. I was off on Thursday, and therefore can't give you an exact reason for your specific flight, but I'll give you another example based on a recent experience. (I'm a dispatcher in the OCC, located in the Dallas HDQ, where such swap decisions are made, and then communicated to the station-level operations where they are executed.)

Aircraft are complex mechanical vehicles, and sometimes (not often) items can break. Rather than arbitrairly ground an aircraft for any failed item whatsoever, there's a FAA-approved document called a minimum equipment list (MEL) that dictates what can be inoperable ("inop") and what (if any) restrictions must be imposed. If an item isn't listed in the MEL, it's a "no-go" item that must be fixed before further flight. As an example using a car, if the car had the windshield wipers quit, one wouldn't necessarily "ground" the car by keeping it in the garage, but instead limit its use to only when it's not going to be raining. Likewise if one's headlights were out, i.e. limiting it to daytime use. Inop brakelights? That'd be a no-go item in any conditions.

The most common MEL restrictions when it comes to aircraft are daytime/nightime operations, or ones that are weather-related, when it comes to the presence/absence of atmosopheric icing conditions, or the ability of the aircraft to make ILS approaches to the lowest landing minimums that are possible.

With all that said, the situation that I had was a flight that was scheduled to operate PHX-LAX, and LAX was forcasted to be near landing minimums in coastal fog. The flight was a "though flight" coming into PHX from HOU, and the LAX passengers that got on in HOU were supposed to stay on during the stop in PHX. I had already released the paperwork for the PHX-LAX flight with aircraft having no deferred MEL items on it. While enroute HOU-PHX, the aircraft had an item fail that (per the MEL) required it to be operating for the next leg PHX-LAX, due to the fog in LAX.

The three basic choices at that point would have been to (1) Fix the item at PHX; (2) Cancel the PHX-LAX flight; or (3) Swap the aircraft in PHX and route it to another destination where there was no fog. If the repair time is too long, or if the cancellation wouldn't have passenger protection on other flights (everything else booked full), then swapping aircraft is sometimes the best choice, which is what we did.

The crew of the "other" aircraft coming into PHX (the one that would now operate the PHX-LAX flight as a part of the aircraft swap) might be having their own "through flight" interrupted, and most crews can readily understand the need for most aircraft swaps. The need for some swaps is sometimes much less obvious, particularly if more than just two aircraft are involved in the swap, or the need for the swap is to head-off a problem several flight segments "downstream" of where the swap is taking place. In any event, with 550 aircraft in the fleet, a crew certainly knows what's going on with their aircraft, but awareness/knowledge of what's going on with the other 549 isn't automatic--the big picture as it were--which is what we look at.

Sorry for the length, and I hope that answers your question.

Cheers...