I’ve had a few avocado trees that have managed to drop all of their leaves. I’m not sure if it was the temperature in the room it’s in, the lack of humidity, a bad nutrient solution, drowned air-roots, or even bugs. Once the leaves get crispy like in this photo, they’re done. The plant itself can come back though.
This one pictured above, I removed all the leaves. Will update in a few weeks with progress, if any.