It's late October and I'm seeing many different bonsai looking pretty ratty, including Chinese elms, Chinese peppers, black olives, Brazilian rain trees, privets and some ficus.
There is nothing wrong with any of these trees, but they present a problem for me because I hate to ship a tree that will come out of the box looking bare. People don't react well to that.
Even the remaining leaves look dull and about to drop. Many of them are about to drop. So what's the problem?
Fall.
Consider the Chinese elm. It can be grown as a deciduous tree, which drops all it's leaves in the fall or, if the conditions are carefully controlled to minimize the changes in available light, it can remain evergreen. But very often does what I'm seeing a lot of this year, it will shed its summer leaves gradually, over a period of a month or so.
Tropical trees can shed leaves as well- black olives, chinese peppers, brazilian rain trees will easily drop leaves in the fall - even ones that have been inside all year, and it might take until the spring for them to come back. Changeing light will do that and so will changing temperature, but that can also be a temporary stress reaction- consider a mini jade moved away from the light source- it's going to drop leaves or a black olive allowed to cool off, those leaves are gone too, but with care and patience they will come back. Let your Pyracantha dry out and those leaves are gone- and I've waited three months for the tree to forgive me and leaf out again.
If you find yourself with a bonsai that is dropping leaves about now and starting to look pretty sad, figure out if you want it tog o dormant for a month or so or if you want it to leaf our ASAP. If dormancy - find a cool, dark spot (do NOT put it outside) Keep it barely moist (NOT wet) and near the end of February, bring it back into the light and start upping the watering.
If you want the leaves back ASAP- just keep it in good light, don't over water (because with fewer leaves the tree needs less water) and watch for the new buds. They will happen.