I see that a big difference in temperature can cause this problem, but I live in Virginia. I had my trees outside this spring, summer and fall and they did fantastic. My orange tree had been inside for a whole year before this summer and didn't grow at all. I put it in a pot and sat it outside and it's almost outgrown its pot. The lime tree lost its leaves before I brought it in, but the lemon tree grew hundreds of buds and flowers. The weather has gotten cold, especially at night. I can't leave them outside or they won't make it. Is there anything else I should do to help them through the winter?
Citrus Trees Losing Leaves
Certified GKH Gardening Expert
The leaf drop can be due to watering issues, fertilizer or disease.
Here are some great articles that will help you.