I'm definitely not experienced, but can fill in the time until someone better comes along
It sounds like a root problem to me. Chinese elms are really hardy but don't like to be too wet for long. Could it be your drainage or soil? Was your mix well draining, did the water almost run right through it? Was your drainage hole blocked? Did the tree sit in water or constant rain for some span of time?
Another possibility is overfeeding. You may have scorched the roots....what did you you and how often did you apply it?
Finally, and probably not a real option, pests. Were there little webs, or ants, or holes in the leaves?
If you applied insecticides, were they all right for Chinese Elms?
Just a few thoughts.
Joanie