This would be the cause of the alarm going off. The alarm panel probably has some sort of output relay, that is triggered on alarm, that is putting a break in the ignition wire. If you post a picture of the alarm I will see if I can tell you what you have if it was built in the last couple of years. I am assuming that you have a hardwired motion, and the switch just shunts the wiring to ignore the change of state at the motion relay. Some alarms do have a key switch zone that will allow you to turn it off and on with a switch, but they are not very common any more.