Ok, I looked back and realized that I had dropped to Gold in 2011, but hit Platinum again for 2012, so maybe it had something to do with that? Here's what the email said when I got it in January of 2012:
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Because you participated in our Exclusive Rewards promotion in 2011—and earned Platinum Medallion status for 2012, you've been awarded 50,000 bonus miles.
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Anyone recognize this?