It looks like Amazon could be the latest in a string of security snafus which have been popping up across the globe the last few years. This one isn't nearly as bad as other breaches. In this instance, Amazon has sent out emails to affected customers that their Amazon account passwords might have been stored or transmitted insecurely, which could have let hackers intercept the data.
Basically, it sounds like Amazon discovered their own security hole and are pre-warning customers. Amazon shared that they were not aware if any third parties obtained users' passwords, but they are forcing them to reset their passwords anyway, just to be safe.
Amazon confirmed they have already corrected the issue, and it's entirely possible that hackers were not able to detect and exploit the problem before they caught it. We expect to hear a press statement at some point from the company. It may be a good idea to change your Amazon password, even if you don't receive an email from the company.
Source: Amazon force-resets some account passwords, citing password leak | ZDNet