Apple is somewhat vague (as to be expected) in its reasoning for this recall exchange:

We recently discovered that some 15-inch MacBook Pro batteries supplied to Apple do not meet our high standards for battery performance. To give our users the best experience possible, we will replace these batteries for customers free of charge.

...however, some users believe that at least part of the reasoning for this is from some recurring battery issue in MacBook Pros that is being grouped in with many of the recent random shutdown complaints. When I took my MacBook Pro into the Apple Store that fateful day just a couple of weeks ago, the Genius determined that my random shutdowns were caused by a defective battery and replaced my battery with a new one. Interestingly however, further investigation reveals that my brand new battery still falls into the serial number range for batteries that are being replaced by Apple, and so I have already applied for (yet another) replacement:

 

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