It thinks a door is open.
The dome light is a simple ground switch. When a door is open, the switch closes and the circuit is grounded-light goes on. Door closed, circuit broken, ground broken-light goes off.
Your alarm's dome light/courtesy light fools the system by grounding the circuit to turn the light on. You can confirm this by disarming your alarm- your door ajar light on the dash should pop on. Flipping the key to ACC will tell your alarm to flip the courtesy light off.
Why do you run 2 alarms?