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IMH (mech eng) opinion, they underestimated the wind and resulting bending moment where the base of the wind turbine entered the ground.
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Not quite..... I’ve been analyzing failures of IWTs for years and have seen many specific modes of failures... at least a dozen unique categories. However, the category that is the most common is fatigue... this can happen in many locations on an IWT and cause a failure but one of the most common locations is with the bolts in the tower flange sections. Take a look at the third picture at post 18... See how the flanges (top and bottom) have separated completely with almost zero damage? It almost looks like all the bolts have simply and simultaneously evaporated and in fact, they have... While the vibration due to the rotating mass could have had a role in the fatigue damage of the bolts, it more likely is just the result of the tower swaying in the wind and putting the bolts through endless cycles of varying stress. Then one bolt goes and puts the rest under even more stress.... however, since they all have been put through the same number of cycles, the remaining bolts are also near the end of their fatigue life and with a bit of an increase in wind that adds a bit more stress, they all start to pop just like popcorn... and down comes the tower. I could show many pictures just like this one... some where the separation at a flange is complete and some just like the picture 3 where it’s still hanging on by a a few bolts that act as the hinge point as it topples over.

Wind turbine failures are on the uptick...
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I suspect that the reason they are ‘on the uptick’ is simply because the number of them is always increasing and because fatigue is a cycle-dependent aspect to failures. Thus the longer they run, the closer they get towards their fatigue limit.

Take a look at the third picture at post 18... See how the flanges (top and bottom) have separated completely with almost zero damage? It almost looks like all the bolts have simply and simultaneously evaporated and in fact, they have... While the vibration due to the rotating mass could have had a role in the fatigue damage of the bolts, it more likely is just the result of the tower swaying in the wind and putting the bolts through endless cycles of varying stress. Then one bolt goes and puts the rest under even more stress.... however, since they all have been put through the same number of cycles, the remaining bolts are also near the end of their fatigue life and with a bit of an increase in wind that adds a bit more stress, they all start to pop just like popcorn... and down comes the tower. I could show many pictures just like this one... some where the separation at a flange is complete and some just like the picture 3 where it’s still hanging on by a a few bolts that act as the hinge point as it topples over.

Looking at that third picture, it appears to me that the steel failed first above the joint, causing the mast with the doghouse to fold and lean over. It was this off-center load that then sheared the bolts as you described, arresting any further folding of the steel at the failure point and instead finished toppling over at the failed joint.

In other words, the bolts failed because the tower leaned over, the tower didn't lean over because the bolts gave out. The bolts were stretched beyond their yield point.

Take a look at the third picture at post 18... See how the flanges (top and bottom) have separated completely with almost zero damage? It almost looks like all the bolts have simply and simultaneously evaporated and in fact, they have... While the vibration due to the rotating mass could have had a role in the fatigue damage of the bolts, it more likely is just the result of the tower swaying in the wind and putting the bolts through endless cycles of varying stress. Then one bolt goes and puts the rest under even more stress.... however, since they all have been put through the same number of cycles, the remaining bolts are also near the end of their fatigue life and with a bit of an increase in wind that adds a bit more stress, they all start to pop just like popcorn... and down comes the tower. I could show many pictures just like this one... some where the separation at a flange is complete and some just like the picture 3 where it’s still hanging on by a a few bolts that act as the hinge point as it topples over.

Looking at that third picture, it appears to me that the steel failed first above the joint, causing the mast with the doghouse to fold and lean over. It was this off-center load that then sheared the bolts as you described, arresting any further folding of the steel at the failure point and instead finished toppling over at the failed joint.

In other words, the bolts failed because the tower leaned over, the tower didn't lean over because the bolts gave out. The bolts were stretched beyond their yield point.

Sorry for the double tap...

After bolting, why didn’t they just WELD the flanges together?

Are they using the highest grade bolts possible or not getting the proper high grade bolt installed?