Maybe a young child. Kids are often excited during Christmas and might be running around the house playing. In their exuberance, they could bump into a table or a shelf where the lamp is placed and break it.
It could be a clumsy cat. Cats are known for knocking things over, and during the Christmas chaos with all the new decorations and people around, it might accidentally jump onto a table and break the lamp.
It might be an elderly relative with shaky hands. They could be trying to adjust the lamp, perhaps to make the Christmas tree look better in its light, and accidentally drop it and break it.