I usually hear about feeders falling apart when people heat them up too much or too fast, but your regimen for preparing frozen feeders is very similar to what I do and I don't have issues. I sometimes get the odd mouse that gets ripped open if a snake squeezes too hard, but it doesn't sound like you're over doing them.

Maybe they were partially thawed prior to you getting them (ie they got somewhat thawed while being delivered to the store) and once you refroze them and then prepared them for your snakes the limbs and skin were more fragile than usual?