Bonsai has falling leaves

From BonsaiWIKI

All trees replace their foliage over time.

Deciduous trees do this annually, and you'll soon become accustomed to their patterns as the leaves turn and fall. Watch the trees in your neighborhood. Your bonsai may be slightly ahead of the schedule because they are in small bonsai containers and more exposed to changes in temperature. Trees measure the day length and decide when it's winter, then the leaves change color and fall.

Even evergreen species, both the needle evergreen and the broadleaf evergreen will need to shed some of their foliage. With pines, you'll see some of the older needles turn brown in the fall, but the newer growth should remain lush and green. You can take a small bonsai broom and go over the tree to help it shed its needles. Sometimes you can pre-empt the foliage loss by plucking the older needles to improve ventilation.

Tropical bonsai will lose some of their leaves too. Watch for the older foliage to yellow and fall. It's no problem as long as the tree is still growing and the leaves that are falling are the older ones.