This is the second time this message has been displayed. I have changed my password and then 24 hours later, Mojang resets my password again. Here is the text:
Due to recent suspicious activity in your account we suspect that your Mojang account email and password may have been compromised. To prevent unauthorized use of your account we've reset your account password. If you use that same password on other sites we strongly recommend that you change your password on those sites as well.