Ficus trees drop leaves to show that it's being stressed, which could be for a number of reasons. It could be that the plant has been overwatered or underwatered. There could also be a lack of light or it's been exposed to a drastic temperature change. Check for any pest infestations causing the leaf drops. If you've just repotted or moved it indoors/outdoors, the ficus could still be adjusting to its new environment.