If you unexpectedly receive a replacement credit card in the mail...

...it means that your bank had been breached, and your account information may have been leaked.

It is much cheaper for your bank to incur the expenses related to issuing you (and anywhere from few hundred to few hundred thousand others) a new card (and/or new account number) - it is a bulk, fire-and-forget process - than to expose themselves to further liability trying to cover things up.

Don't forget to update your recurrent payments, and keep praying that it's just your card or account number have been compromised, and more substantial pieces of data such as your mother's maiden name or last four of your SSN. Good luck.

UPDATE: If they're nice, they let you know they've been breached. In California, they are required to be nice by law.