My 25 year old potted ficus will do this from time to time. Usually a sign that it needs additional water or repotting. Check the pot. Can you dig down 4-6 inches? Is the soil damp? then water the plant well and two days later check again. Is it wet or dry in the hole? the plant could be so root bound that the water is running around the sides of the pot and not to the root ball.
Check for red spider mites using a piece of white paper under the leaves. Tap the branch and see if there are any red dots on the paper.
Are the leaves sticky? It could be aphids and ants.
Check with a certified nurseryman. Bring in a bagged sample of the leaves and see what they have to say before you treat.
BTW, I repot my ficus once every three years. Out of the pot, remove the soil, trim the old roots, back in the same pot with fresh soil. Plant no deeper than it was planted before. Water well and stick it back in the same place as before. Ficus don't like changes so don't move it to any new place-that is sure to cause leaf drop.