It was most likely a build up of hydrogen sulfide and carbon dioxide
from organic materials in the pond. Our ornamental ponds have way too
many fish per amount of water. Even large natural ponds can have winter
die off if they are frozen over for too long.
Here in zone 7, and relatively mild winters (only a week of being
frozen over and snowed in usually) I use an air pump, line and air
stone to keep a hole open in the ice. If winter sneaks up on me, just a
day late in getting out to the pond I can have fish die in 3,000
gallons.

If this is not possible for you in your climate you might consider
reducing your stock, draining and catching the fish in the winter and
keeping them in a stock tank with filtration and air pumps over the
winter. Some folks get really carried away and set up a whole
wintertime pond in the basement ;-)

kathy :-)