From the photos, it looks like a classic fatigue failure to me, affecting all the bolts. If so, it is an age-related failure as it takes a certain number of stress cycles to grow the fatigue crack across the bolt to the point the remaining intact area is not strong enough to withstand the tensile load and the bolt fails.
A characteristic of fatigue failure is the fracture surface shows a relatively smooth area with fine, often crescent-shaped lines spread across it. The remainder of the fracture surface has a rougher crystalline appearance, which is caused by the sudden final tensile failure.
Bob, can you post a close-up photo of the fracture surface, the closer the better?
Malcolm
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