Ripsgreentree can probably confirm this, but the root system appears to me to have developed this way in response to growing in a nursery liner and then being transplanted on without much care given to the root system. So the roots tend to swirl and grow down.
Two ways of dealing with this are to treat it as a defect and correct it, or make it a focal point of the tree.
If you decide to live with it, the most detracting portion (that curving serpentine root that appears in the photo in the 10 o'clock position, could just be cut away.
If you would like to look into improving the root base, you could do something called a "ground layer" which amounts to an air layer right in the bonsai pot. It sounds drastic, but with a plant like elm, it is 99.9% effective.
[*]Chalk a line around the trunk where you would like to have roots develop[*]then using sharp knife make a cut around the circumference at that point all the way down to wood.[*]Make another cut a generous distance beneath this one. Typical distance would be 1/2 to 1 x the thickness of the wood. An inch or so would be good. Cut all the way down to the wood again.
[*]Now with a knibbler or pair of pliers, remove all the bark and cambium between these two points.[*]For best results, swab the area with a rooting hormone. For elm, it's practically unnecessary.[*]Place several wraps of large gauge wire immediately beneath the upper cutline and tighten with pliers to prevent the tree from trying to grow over the wound. We want it to issue roots.[*]Next step is to build up the side of the container so we can have enough soil to cover the wound. I use some drainage cloth a few inches high, form it into a fence and stake it with a few pieces of wire. So now, you have a little retaining wall.[*]Fill with bonsai planting soil to a point at least 1" above the cut area.[*]Now, the hard part - wait about six months and you will have a brand new, practically perfect radial root system. Try not to check more frequently than every 2 months.[*]Rotate the pot to keep the sun hitting it on all sides.
Check it in fall, for an elm you may be able to separate the layer at that time, but for best results wait 1 year. Many trees take 18 months to 2 years. Elm layers quickly. Wisteria makes elm look slow!
Check this site for some photos. Keep in mind that this technique shown here is an air layer, but it shows the basic process. It's a good article, but don't confuse superthrive with rooting hormone. They are different.
Regards,
Matt