Critical Paths
Catastrophic engineering failures tend to follow certain critical paths. One path is multiple failures, which each alone would not cause the catastrophe, but in combination, in a specific sequence, under specific conditions can lead to the failure. This is sometimes known as the cascade effect.
While it is certainly too early to speculate on what caused the Columbia failure, I would not be surprised if it isn't a series of events that may have occurred before, but not in the sequence, or to the extent, required for the failure.
I would not be surprised that the failure began earlier than NASA thinks it may have.
I would also not be surprised if some of it is related to the low level of funding NASA has endured over the last 20 years or so.
Whichever the case, the investigations will begin, reams of reports will be made, and I hope, the shuttle or its replacements will fly again.
Aloha!