Cause of Ulmus Parvifolia leaf drop?

I have several ulmus parvifolia specimens in my collection and had a new problem several weeks ago.  One of them, a pre-bonsai I've had in my possession for about a year and a half,  suddenly dropped its leaves.  At the time they dropped, all leaves were green and outwardly healthy, no sign of black spot, dryness or other problem.  They all just dropped in the course of a day.  My other elms, all of which I have had for a much longer time, were not affected.  They are all in similar soil, have similar watering and fertilizer regimens, and were, up until the leaf drop, on the same bench. I isolated the plant, moved it into dappled shade, and a new crop of leaves has begun to sprout.  

Below are a couple of  pictures. Sorry they're sideways. Anyone have any ideas?  I sure don't want it to happen again.

Is anyone else struck by the fact that in the past month or so this is about the fourth thread we've had about problems with Chinese Elm leaves dropping?

I really do suspect that our worldwide rather bizarre weather patterns have played some, possibly even quite a significant part in this.  What has your weather been like over the Spring and Summer, timahlen?


Glad the new leaves have come back in. FWIW this is exactly what happened to two of mine.

We are in Zone 8, Dallas, TX. The weather has been erratic. July was unusually wet with high temperatures in the mid-70's for a couple of days. By the end of July, temps were over 100, and it has dried out.

Temperature changes have been sudden -- both up and down -- here in the Southern part of the USA. I think some of our trees have a hard time coping with the yo-yo effects -- tho it hasn't show up in my elm, yet. Here in the SE USA, it has been rain that's the biggest problem -- too damn much of it.

My non-native juniper are all complaining -- as are my 2 Ficus.