When strings of lights hang around the tree and they're left on for any length of time, magnetic fields build up around the entire circuit. It might not seem much, but over the hours that little trickle is enough to bring life to those dormant angels hanging on the tree alongside them.
The angels on the tree are ready, believe me. They'll move in the dead of night, sliding silently from room to room until they find their prey.
Did you switch the lights off? Are you sleeping comfortably?