Also, there are only 3 hard drive manufacturers left. If one of them have a bug that affects across their product line, that can take out 1/3 of all hard drives.
How many years between incidents are you talking, and when was the last time a manufacturer had a multigenerational bug? (and is quality improving, or decreasing? That is, are we more or less likely to see failures in the next 10 years than we did in the last 10?)
Disk failures could also be triggered by datacenter environmental factors shared among many drives like temperature or noise.