In many cases, UA will put a bag onto the next flight to its onward destination, whether some future flight is full or not. (The passenger cabin can easily be full while there’s still plenty of room in the cargo hold).

While it’s certainly possible that your bag was on your flight and made the tight connection — the bags typically have a shorter path to take, and they can be loaded all the way up until pushback, whereas passengers can only be loaded until the door closes — it wouldn’t be unusual for UA to have loaded the bag onto the next flight out either.