One possible theory could be the way you are taking the discs out of the cases. Maybe they are stuck in quite firmly, and you are bending them to get them out, and thus, over a period of time, weakening the centre? You said you used them a lot, so maybe getting them in and out of the cases caused some damage? Just a possibility, although I have never heard of a disc 'exploding' inside a drive before. You were very lucky to be able to clean the drive out, and it remained usable!