Certified GKH Gardening Expert
There would be many things that could cause this. Typically when stressed, a fig tree will first drop its fruit, then its leaves - but it can happen the other way around.
If it is in a container, my first thought would be it is likely not getting enough water. And even if it is in the ground, many places have experienced a very dry summer, so if this is true for you, a tree that did not need watering in the past may need extra water now due to low rainfall.