Several conditions could lead to significant leaf loss of your Madagascar dragontree (Dracaena marginata). Tropical foliage plants are often grown in bright greenhouses or outdoors under shade cloth. In either case, the plants are accustomed to much more light than you can provide in the brightest of spots in your home. As the plant adjusts to reduced light conditions, it often sheds some of its lower leaves.
Another possibility is that your plant might have gotten a chill on the way home from the store. Tropical plants such as dracaena react dramatically if they have experienced cold temperatures or drafts. Make certain that the store associate wraps your tropical plants well to prevent them from getting cold in transit. Preheat the car before taking the plant outside, and drive directly home. Avoid the temptation to stop off for another errand; a car cools off quickly in cold weather. Too much water could also be causing leaf loss. The soil should be allowed to dry to the touch between waterings. If it stays constantly wet, the roots could begin rotting. Yellowed leaves and leaf drop follow.