You may be overfertilizing, over or underwatering or your tree may be too close to the light source. What type of light are you using?

I'm no expert and do not have to take my trees indoors in winter, but I lose no trees unless I yank them myself :-)

A lady next door to me asked the same thing of another plant. I told her the first thing to do is look at what you're doing, and stop to evaluate that.

She thought she was not watering enough, and watered even more, and it killed her plants. The same applies to the opposite, if the leaves curl or wilt, get brittle and drop, they're likely too dry.

I fertilize sparingly especially in winter, when I stop for most everything. I think over fertilizing is worse than under fertilizing. Under can be addressed faster than over.