So, when you connected to the city line, you put your system under city pressure. At that time, perhaps you put on your hot water tank? Maybe you did, and perhaps the pressure was high on the city water and rather than expelling the excess once the water was heated out the pressure relief valve, it found another place with just a little less pressure required.
It's a possibility for sure. You can try it again, and then try pumping up your own system and then leave the taps tight and start the hot water tank to see what happens.