There's no "turned out to be" about it - many of the actual mortgages were known to be sub-prime from the start, that's the point. The risk was knowingly taken - however cavalierly, knowingly.

The problem arises then because of the housing market slumps (mainly in the US) being far worse and for far longer than expected. Because of the "slicing and dicing" no-one knows where the risk now is - and not all of it has yet come out of the woodwork because the defaults didn't and won't happen all at one go. There MAY still be far worse to come, but no-one can tell whether, when, or above all WHERE. That uncertainty is what is paralysing the inter-bank lending markets and thus causing the greater problems than the 'headline' figure for actual debt so far revealed would otherwise have done.

WW