Chinese elm dropping leaves quickly

I have various kinds of Chinese elms in containers that I have been growing for many years including regular Chinese elms, yatsubusa, cork bark, seiju and hokkaido. All of them leafed out nicely and then suddenly over the last few days a few of them started dropping leaves at an astonishing rate. Mostly all green leaves. Some leaves have turned a bit yellow and some show small areas of discolorations and some show a few dark spots too. No leaf deformations. This happened last year too with some of the plants. The plants sulked all season last year but all came back nicely this year. I do not see a pattern with any particular plant. It seems random.

The only thing I can suspect is it is a fungal infection of some sort. Google search showed that elm anthracnose could be a culprit. But the symptoms are nowhere close.

Any idea what is going on and how to correct this?