Here's a theory.. Aren't many insect eggs able to withstand extreme temperatures? Maybe the rat already had fly eggs on it. Maybe the rat sat in a pile of dead rats just long enough at the rodent place to have a fly lay eggs on it before it was frozen. Then the eggs went into a sort of stasis. The snake ate it and it may have either been a little large, or possibly had begun to rot slightly before it was frozen which caused the regurge. Viola. The eggs hatch because they were now warm and had not been in contact with the stomach acids long enough to destroy them?
Possibility?