I don't have any idea what might be wrong with your trees, although I do wonder if you're watering too much. Watering an established tree every couple of days, even during a drought, seems excessive... a once-a-week, deep soaking would be better for them than more frequent ones. I don't know if that's what's causing the bark to fall off or not.

Emerald ash borer is a huge problem in some areas -- I know you don't suspect borers, but you might want to take a look at this article (link below; scroll to bottom for information on symptoms) and then look closely at your trees, just to make sure.

Another source for help is your local cooperative extension service, and/or master gardener program. Copy and paste this link to find out locations, etc:

http://web.extension.uiuc.edu/state/

Some states have a pest and disease diagnostic lab as part of their extension service -- great place to get help.

I hope your trees are ok -- good luck!

Here is a link that might be useful: emerald ash borer

Jena-I wonder if you ever diagnosed the problem?? My new green ash is losing bark, as well, and I don't have any of the borer symptoms, either!