My condolences
You've had that baby for so long and were no doubt very much attached. As to what happened ?
It "seems" to be somehow connected to the last feeding, but without a necropsy its all just guess work.
Were the mice snacking on poison before being sold? Were they sick? Did something go wrong with your BP killing and eating them? It could be any of those, or the temp fluctuated or he developed a illness.
You did what you could and you gave him a good life. That is all we can do at times.
Don't beat yourself up, your snake had so much more then so many of them have.
Take your time to grief, and then give another little snake a good life. That is one good way to honor the life of the baby you lost..