the chlorine content in city water should not be high enough to cause critters to head for the shore? If I take chlorinated tap water and put fish/frogs in it they don't seem to be affected. Also, assuming the chlorine in the treated drinking water was consistent at the time of the break, when it hit your pond it had to be very much diluted by your untreated water.

I would have to say some other chemical event or later event (bloom?) had to be the culprit but what?

I guess you would have to see if something else mixed with what was in the water main as it was travelling to you, such as a toxic oil or something?