Sporadic watering is more likely the issue. Drying out a bonsai plant will lead to leaf drop. Plants do have a natural rhythm on when they need water, it slowly increases in spring and slows down slowly in late summer/fall. Once, you figure out the rhythm it becomes easier to predict when to check to see if they need water. Remember bonsai dry out much quicker than most plant. They need a lot of water, because they are in shallow pot with extremely well draining potting medium.
If you have difficulties knowing if it's time to water, use something as simple as a bamboo skewer. Insert it in the pot. The next time you think it is time to water, pull it out and check it. Look at it and feel it. If it is wet it is too early, if dry its too late. If the stick does not appear wet, but feels moist it is time to water.
You also do not put your grow light on long enough. The general rule is 12-14 during dormant season and 16-18 during the growing season. This plant does not really have a natural dormant season, so you should double the amount of light you are providing. You do have a window near-by so you do not need to go all the way to 18 hours, but you should not do anything less than 12. The light should also be about 6-8" above the plant. Also, fluorescent grow light bulbs should be replaced every 2 years.