Composite structures are hard to examine. You can't see defects visually. Instead, you have to use ultrasound to find signs of voids or delamination. According to reports, one of the six vertical stabilizer attach structures showed signs of delamination when the plane was being prepared for delivery back in 1988. Airbus implemented the standard repair technique and no further problems were noted.

This same plane was involved in a severe turbulence incident about 7 years ago that injured 40 people. It's possible that the vertical stabilizer was somehow damaged during that weakened the structure. The plane went through an extensive maintenance cycle a few years ago and the vertical stabilizer was examined then. Still, I can't help but wonder if some type of damage was present.

Other possibilities include things like improper curing of the composite components during manufacture or that the structure somehow became brittle over time. If the unit had been damaged in the past or if some other defect was present, it's possible the component couldn't withstand the forces of wake turbulence and the crew's control inputs.

I'd be interested in knowing if the FDR data from that earlier turbulence incident is still available. I suspect it is in an NTSB archive. I'd like to know what kind of forces were put on the vertical stabilizer then.

Aircraft are designed with a 50% safety margin. That means that if a plane is certified for a given G load, such as 3.2 Gs, then it has to be able to withstand that load without any permanent deformation or damage. However, if that load limit is exceeded, deformation begins and the structure is weakened. At the ultimate limit (4.8 Gs in this example), the component will fail. However, if the rated G limit was exceeded before, then the ultimate G limit is decreased because the component isn't as strong anymore due to the deformation.

I don't know what the lateral gust load limits are for the vertical stabilizer. I do wonder if those limits were exceeded before or if they were exceeded earlier this week. I suspect a lot of aerospace engineers that the FAA, NTSB, and Airbus are looking into that issue right now. Airbus and Boeing both make extensive use of composite structures in their planes. This week's crash could be a one-time freak incident or a warning of problems that may exist in many planes.