When the fire horns aren't being used, the wires are constantly checked using electrical current. When the fire horns are turned on, when they get current. Reversing polarity is the only way to turn on and off the fire horns.
A fire horn is turned like a light; it turns on when power is applied and turns off when power is removed. But then again, to constantly check continuity of the wires (supervise the wires), the Fire Alarm Control Panel (FACP) always has power applied to the circuit.
This is a conflict. Only when there is an alarm are the horns supposed to make noise, but just to check continuity of the wires, the panel has to apply power all the time.
To prevent the fire horns from making noise while the wires are being checked for continuity, the voltage for the continuity check (supervision) is reversed. The horns, then, have a diode inside them to block the electrical current, so the horns stay silent.
It's an automatic turn on/turn off. When there's an alarm and the panel wants to turn on the horns, the panel changes the Notification Appliance Circuit (NAC) voltage to forward polarity. Once the voltage is forward, the diode in the fire horn allows electrical current to flow and the horn turns on, making noise.
That's why the voltage reverses when the horns are turned on.