Hypoxia. This happened on Ball Pond a few years ago when they stocked through the ice at the boat launch where the cove is very shallow, full of dead weeds, and is the first spot to freeze and the last to thaw. Ideal conditions for hypoxic conditions where all of the dissolved oxygen is used up by decaying weeds, etc. and a rough winter that allows no oxygen replenishment. I made arrangements for the stocking truck to use a vacant private lot on the west side of the pond where the shore drops off into 20 feet of water and into the main basin and this is now the new stocking location for Ball Pond. It also gets the trout into deep water so that the comorants can't herd them into the corners of the shallow cove and gorge on them.
I know the location you are referencing from being on the stocking crew a number of years ago and normally in open water there are no problems but this year nothing should have been stocked there through the ice. The only stocking locations should have been near deep water like Squantz Pond state park and Lynn Demming.
Inland Fisheries needs to know this happened.