If the amount of water was substantial, over 5% to 15% of your pond's volume, you could easily experience poisoning from city water purification additives, e.g. chlorine. There is also the possibility that the city water temperature was considerably different than your pond water temps causing temperature change shock (unlikely because city water is typically around 55 degrees F. and your pond water should be close to that this time of year)or even a sudden turn over in the pond. If the temperature difference was greater than, let's guess, 15 degrees or greater, a quick turnover could occur given enough city water got into the pond. Once again the amount of water would have to be large compared to the pond volume and/or drastically different in temp.

If the flow through the grass was slow enough, the chlorine could likely dissolve out reducing the amounts of toxins into the pond. I am betting you got a lot of city water into the pond and chlorine and temperatures are your likely culprits.

I am guessing that city water is also low in DO, or maybe high, if either is the case , and enough water got into the pond, a drastic change in oxygen levels could cause shock/stress on the pond's inhabitants

I am basing this on years of tropical fish aquarium care, not necessarily pond ownership, but the concept is the same and aquarist are always contending with city water and very concerned with temperatures when doing partial (15-30%) water changes.

Fish on!,
Noel