I had a similar issue, but it was a single RAID-5 array and wear of some other manufacture defect. They were the same brand, model, and batch. When the first failed and the array got in recovery mode I ordered 3 replacements and upped the backup frequency. It was good that I did that because the two remaining drives died shortly after.
The lesson I learned is that the three replacements went to different arrays and we never again let drives from the same batch be part of the same array.