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Each antenna you have retains some sensitivity to stations "from behind," so your NYC antenna is picking up some small amount of signal from Bridgeport and vice versa. Apparently, under normal circumstances, there aren't enough "other market" signals in each antenna to cause problems. Normally, we do not recommend combining antennas the way you have because it is hard to predict the outcome.
The issue is how well in phase the signals combine. There are two sources of phase delay in your setup. Focussing on the Bridgeport direction, the signals from that direction arrive at the HBU-55 later than the DB-8. Those signals then have to travel back down the coax to the reverse splitter, adding more delay. (The reverse applies to the NYC signals, where they arrive later at the DB-8.)
The degree of phase difference between the signals at the splitter depends on frequency (as frequency increases, wavelength decreases - so a fixed physical distance is a different amount of wavelengths at RF 49 than at RF 42, for example.) So, for some channels, signals from the two antennas will add in phase (good!) and, for other channels, signals from the two antennas will add out of phase (bad!).
Again, under normal circumstances, you seem to be OK. However, when propagation conditions change (and they will from time to time), the NYC antenna is now getting enough signal strength to cause problems to the Bridgeport channels. Note that WEDW is the strongest station from the Bridgeport direction, so the likelihood of getting some signal in the HBU-55 is greater than for others. It also means that the spacing between your two antennas probably leads to destructive signal combinations at the splitter.
The same argument in reverse could be made for WLIW. Remember that the effects, positive or negative, are frequency-dependent in this scenario. The only way to test for sure is to remove one antenna from the splitter and replace it with a terminator.
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