After some recent travel, I noticed my Mileage Plus card was being declined (although I had called ahead of time to inform Chase that I would be traveling). When I called customer service, they did see the note in the system indicating that I would be traveling, but they stated that my card had been "compromised" and would be terminated. Supposedly, a new card is coming my way.

Since there were no bogus charges on my account, the problem was not fraudulent charges. All I can think of is that some number of cards were in a database that had been hacked into.