if a group of cars is parked in a tow away zone the tow company can tow all or one of the cars that is in the tow away zone. They are not required to take them all. The tow truck company would have to be notified if a car in a tow away zone. So most likely if all cars are in a tow zone and just yours got towed, someone most likely called it in to the tow company.

in your case someone most likely called and complained because your car was most likely in front of private property.

in short they can tow just one of the cars even if there is no sign.