It is caused by a mold that if found in poop of pests.

Most indoor Bonsai species (Ficus, Carmona, etc) do not drop their leaves, unless there is a problem: Overwatering is a common reason when the indoor Bonsai is planted in poor soil that retains too much water. A few months of overwatering will lead to root-rot, only visible in the tree losing strength and dropping its leaves.

This can lead to water stress which can be lethal to your bonsai tree.

May 02, Factors that can Make a Bonsai Tree Lose Leaves (+ How to Fix the Issues) 1) Overwatering or Under-watering. Both overwatering, as well as under-watering, can lead to leaves falling off with ( 2) Exposure to Higher/ Estimated Reading Time: 6 mins.

Jun 09, Most indoor Bonsai species (Ficus, Carmona, etc) do not drop their leaves, unless there is a problem: While with overwatering the tree would slowly lose strength, with under-watering the tree will drop its leaves within days. >> Click to read more bonsai leaves.

Sep 03, When a tree adds as much as it drops, it has no reserves and no energy to build more. It's pure survival for the sake of survival. To add new foliage and basically stay alive, it sucks the older foliage dry and recycles all resources it can get out of it.