Clustered failures is just the way durability failures work. It doesn't mean that the tires are bad or the tires are over-loaded. It means that's just the way failures occur.
For example, if I take a bolt and pull it until breaks - then do that for 100 different bolts - I will get a cluster around a certain value.
If I change the bolt to a stronger material (like going from a grade 5 to a grade 8), the test results would be higher but still clustered.
So when a person experiences multiple tire failures many years after installing them (with no incidences before that), all it means is that the tires are reaching their durability limit. This could be because the tires are overloaded (or under-inflated) or it could mean the tires don't have very good durability. You can't tell which it is without additional information.