There are a couple of things it could be. Usually dropping leaves en masse is an indication of a larger health problem. It's hard to tell with just a single picture, but based on my experience I think this is part of the standard annual cycle.
Weather has been crazy in recent years, but looking at my own yard fruit trees have started flowering etc. etc. so Spring has already arrived here in the Southern hemisphere. I am guessing in the north the season is also turning into Fall.
During Fall trees have a secondary growth spurt similar to what is seen at the beginning of Spring. With Elm trees this is typically expressed as new branches growing. An Elm tree typically grows a bunch of these new thinner branches with new leaf growth and during the dormant period over winter about 50% of these new thinner branches die off. So when shaping a bonsai you end up having to shape and train every year.
What I am trying to say is, with an Elm you get more branches and need to do more work with training as opposed to other trees where you get less new branches and more of a typical hierarchy.
So, with a happy healthy tree at the beginning of Fall I would be expecting to see a secondary burst of new thin branches with leaves and a gradual dying off of existing leaves which were born during the previous Spring. Also, as Elm trees are deciduous, by the end of Fall your tree will appear to be completely dead with no leaves left on its branches.
The typical cycle for an Elm would be as follows:
- Spring arrives: New growth comes
- Mid-Spring: Wait until majority of growth period has finished then trim away any dead branches. Re-shape and prune as necessary to get the shape you want. Note, you can prune as you like, but do not prune major branches and if you are pruning branches with leaves on them you have wasted all this energy, these branches should have already been considered in Winter pruning
- Late Sprint: Growth should have finished and like a person a couple of weeks after a good haircut the plant should be looking at its best. Now is the time to put the plant on display
- Summer: Plant retains its shape, no major growth. Enjoy looking at your plant and ensure it has sufficient water and doesn't get scorched.
- Early Autumn: Second growth spurt begins. Start to notice signs of yellowing. Do NOT trim new growth or shape at this time as your plant will be bare in a few weeks and you are wasting your time trying to make your plant look nice.
- Mid Autumn: Second growth spurt stops and plant will have dropped about 50% of its leaves. Remove plant from display.
- Winter: All leaves will have dropped and plant should be dormant. Reduce watering. Now is the time to plan what you expect your plant will look like when it comes back to life and consider what branches need the chop. Prune your plant as necessary. Now is a good time to do any repotting or structural rewiring.
As an aside, you can actually trim new Fall growth, but you may not get much redirection of resources. This is personal preference if you think that you may get growth to appear elsewhere, but IMO it's not worthwhile.
All of the above aside, maybe you think I'm wrong and you think there is actually a health problem and it's not part of the natural cycle. That may be the case, I didn't go there because this answer is already long enough, so let me know if you think I am not right and I will go down the other path and what that means. In a separate answer.
Last thing I would like to mention is about the location of your plant. I am not sure if you are keeping it inside permanently or just for taking this picture. If you are considering keeping it inside I suggest you check out my answer in another question here as I suggest you reconsider keeping it inside all the time.