Why does overspeed cause structural damage or failure when limit load factors are not exceeded? The positive and negative limit load factors in this diagram are around 4.5 and -1.8, which means the aircraft experiences structural damage when the aircraft is flown beyond these limit load factors, and beyond the positive and negative ultimate load factors, which are around 6.6 and -2.8 respectively, the aircraft would experience structural failure. These excessive loads typically occur when the aircraft is maneuvered abruptly as in steep dives and steep turns or when the aircraft is flown in turbulent air.

I understand it's excessive loads that cause structural damage or failure when an aircraft is flown beyond the positive or negative limit load factors. But what induces structural damage or failure when the same aircraft is flown within the limit load factors? 

My best guess is the parasite drag, but the fact that the normal operating range (green) within the limit load factors jumps to structural failure (red) without going through structural damage (orange) leaves me at a loss, because if the cause is the parasite drag, I don't see the reason structural failure should occur without the structural damage being first experienced. 


Edited in response to Simon and kevin's advice
Flying way too fast is an example as shown by the graph. For example descending with full power while maintaining 1 G load factor. Or with a powerful enough engine, exceeding the speed limit in level flight.

The air impacts dynamic pressure on the aircraft, the faster the plane, the higher this pressure—up to the design limitations.

What causes structural failure when the aircraft is merely flown fast?

High speed induces vibrations, flutters, etc., all would likely cause loss of control and/or structural damage/failure.


  Flutter: At its mildest this can appear as a "buzz" in the aircraft structure, but at its most violent it can develop uncontrollably with great speed and cause serious damage to or lead to the destruction of the aircraft.


(Source) Flutter.