Motion detectors can be quite sensitive to even the smallest movements. The alarm is triggered by the change in the amount of light in their area. But from what I know, motion sensors have been modified in a way that it'll only trigger the alarm when the change or motion is really discernible. Small actions won't trigger it all of a sudden anymore.
and I'm confused, too, how the alarm can go off when it's enabled. Perhaps the problem is with the other devices? Do you have other security devices (glass breakage, gas alarms, etc) installed?
"L" ~ Home Security System