I would think your water had already turned over, so that acidic, anoxic water was no longer in the bottom half of the pond....If your aeration diffusers are set deep, and your pond was iced over, you may have supercooled the pond...we need to know more info to offer better opinions...Honestly though, it sounds just like a mid summer aeration start up that mixes the poisonous bottom water into the rest of the water and kills fish.