Municipal water supply usually have Chloramine added. Chloramine bind with the
hemoglobin in the blood. This prevent the transport of oxygen by blood
circulation. The fish would die due to lack of oxygen. Chloramine from that
running hose killed your Koi.

CC wrote:

One day, we were trying to get the dead leaves out, and toward the end of
the day a garden hose fell in the pond. I did not noticed.

The day after, I found 28 dead kois (7" to 12") splashed around the
overfilled pond and one lone small goldfish alive inside.

The hose was spraying full force and the pond was crystal clear.

I was devastated! I cried like a baby. These kois had become my best friends
and my morning ritual. To this day I can't go by the pond without feeling a
knot in my stomach.

Being new at ponds and not knowing any better, we (my wife and I) went out
and bought 12 baby kois (we had the water tested beforehand) and repopulated
the pond.

I have 2 questions: 1) What did the kois died of?