Leaf-drop can occur on Portulacaria afra for a number of reasons. As discussed in your workshop, following a repotting in-class, your bonsai might want to redirect the sugars and other resources stored in their leaves elsewhere while the roots establish and fresh cuts callous over.
Occasionally, this species will shed older, interior leaves as part of its natural cycle. It is perfectly healthy. In the wild, these inefficient older leaves will recycle into the soil. It is not suggested to leave any decomposing leaves on the surface of your bonsai soil. These leaves will attract pests and unwanted mold and fungi to your tree.
During major seasonal change when the amount of sunlight decreases; your bonsai will alter in strength can also prompt the shedding of leaves. In Winter, the tree might develop flatter and wider leaves more suitable for gathering as much solar energy as possible. Winter is a time to consider supplemental lighting in the form of an LED grow light.
Sapsucking pests and certain molds or fungus can also weaken, corrupt, or drain, leaves, causing them to fall off. Inspect your tree for insect pests or foreign substances that might be the source of the problem.
Most commonly, if the leaves across your tree are shriveling and many are falling off; the issue is likely underwatering. If you are unsure about your watering, refer to our watering article and related video. If the pads of your bonsai are remaining mostly full, but are discolored and squishy when falling off, the tree is likely being overwatered. Please refer back to our article on watering for information about proper techniques and methods.
Leaf-drop of healthy leaves might also happen from impacts, vibrations, or other physical stress like strong winds. Consider if the environment of your tree and the physical conditions if healthy pads are snapping off at the base.