It doesn't seem to be recorded, unless Tolkien addressed it in his letters. They were all ensnared around the same time of the Second Age, and I wonder if they were not all allies or sub-cheiftains under King Angmar. Perhaps it was he who distributed the other rings, promising long life and power to them in exchange for their help in his wars.
Of course, that's just speculation. I have read nothing that would confirm that.
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