Where do you have your tree located? It may be trying to go dormant.

Has it been watered properly?

What species? Alii? Nerifolia? Salicifolia? First, your tree will need very little water when it's foliage has been reduced substantially and abscission is still occurring, so water only when the soil at the drain hole(s) is dry to the touch.

Tropical Ficus spontaneously defoliate most commonly as a response to a decrease in light levels, as a drought response to soil being either too wet or too dry, or exposure to sudden chill. All of these conditions cause a reduction in the flow of auxin (a hormone/growth regulator) through leaf abscission zones. A steady flow of this hormone is required to keep an abscission layer from forming at the base of each leaf petiole (stem). Interrupting the flow = leaf abscission (loss).

Plasmolysis could also be a suspect. We refer to it commonly as fertilizer burn, and it can passively occur when plants are moved from high light and usually warm conditions, with high concentrations of fertilizer in the soil (like you would find at the growing facility), to the low light and usually cooler conditions you might find inside a big box store or your home. This condition is less likely though, if your tree seems to be pushing new growth.

Suggestions: Water as indicated above - keep the tree above 55* F. - give it as much light as you can - sweep up the leaves occasionally ;o) - wait - the tree probably has plenty of energy reserves because it's a recent acquisition, and will likely produce a new flush of growth with little trouble. It is important, however, to protect/preserve the emerging flush of leaves because they will be an energy sink (they will draw on the tree's energy reserves) until new foliage is about 75% mature - and the tree will almost certainly not have the energy reserves needed to push an additional flush if these are lost.

Al