Could be either or; could also be a change in photo period (shortening of days) or a change in light conditions (you moved the plant to a new locale). Doesn't matter which of the above it is; in the immortal words of the Hitchhiker's Guide to the Galaxy, DON'T PANIC! Leaf drop is as common as rain in ficus and is usually followed by a new (smaller) set of leaves (just like you want in a bonsai) In fact, an old method of getting a ficus ready to show is to remove EVERY leaf six weeks or so before judging, leaving you with smaller, fresher leaves on the whole tree!
So take deep breaths, Richard. Relax. This time of year it's o.k. to skip a watering here and there. If a few leaves drop it's just nature's way of cutting down transpiration (breathing) so the plant uses less water (either from too much OR too little). Just think of it as your tree taking care of itself
. Ficus are tough trees; mine is twenty or so years old and I've run it through the ringer and it still looks great; yours will too...