A healthy Willow leaf ficus can drop all of their leaves during winter, happened to myself last winter and several ppl mentioned in this thread from last week
https://www.bonsainut.com/threads/ficus-defoliating-on-its-own-under-lights.67404/
What I would be concerned about is if you are getting die-back of branches at the same time.
One problem with having trees sit in a tray of water is having the soil constantly damp, which is something you want to avoid. Not sure if that is the case, as something I can't tell just from one photo.
Changing the amount of light the tree receives, like if you did a recent reshuffle of the position of your plants, or if your tree is not receiving enough light could also cause it to drop its leaves. Do you feel the tree is getting enough light? Did you have it growing under this light last winter?