I would say it's the POM. I've never had any just die from bad genetics or anything like that. They're pretty hardy. Those mice could have grabbed a piece of bedding as they always do, and chewed on it, getting the poison into their system, and killing them. I would not use that bedding if you've sprayed it all.

Thoroughly clean out the tubs to get rid of any residue, even in the rat cages, and replace them with clean bedding. If you need to, go to a feed store or another similar store and pick up Zodiac Kitten/Puppy Powder. It gets rid of fleas, mites, etc and is very gentle. You can even use it underneath your snakes' bedding as a preventative measure.

Also, you can treat every rodent that comes in your door with a small dose of Ivermectin paste(1.87% horse paste, usually costs around $5). Dose is an uncooked grain of rice once a day for 2 weeks(or until fed off). They should do fine with it and it won't affect the snakes any.