If you thawed it properly it could have fallen apart due to being partially thawed during shipment (ie from the supplier to the Petsmart, etc) - it weakens the flesh when someones goes to thaw it for feeding and falls apart. Probably a good thing you tossed it, I've had feeders' guts fall out after being squeezed by a snake and that's a mess.

Do you have a mice/rat breeder in your area to save you money on feeders?