Sony doesn't really explain the irregular activity.
In response to detecting "irregular activity," Sony has reportedly reset the passwords for some PlayStation Network and Sony Entertainment Network accounts. For those affected, the company is sending out emails, which were reportedly received by two account holders at The Verge.
"The Sony Entertainment Network team routinely monitors for any irregular activity, and if such activity is detected, we may sometimes reset passwords of affected accounts to protect consumers and their account information," the email begins. "Your account password was recently reset as part of this process and you will need to create a new password the next time you access your account using the ‘Forgot Your Password’ option on the Sign in screen."
"We encourage you to create a complex unique password that you have not used before or isn't the same password as those associated with other online services," the email continues. "As always, we also encourage you to keep a close eye on your account for any unusual activity including emails about transactions you did not perform. Please click here if you need help in changing your password, or to contact us for additional assistance."
"Also, remember that Sony will never ask you for your password or any personal or account information in an email," the email concludes. "Please use caution if you receive any emails or phone calls from anyone asking for personal information or directing you to a website where you are asked to provide personal information."
Sony doesn't go into specifics about the "irregular activity," but the company is understandably cautious given all the recent press about the NSA and the April 2011 PSN hack that left the network down for weeks. There's also no indication of how many people were affected by the "irregularity." However, the Verge lists two factors that may have pushed Sony to reset passwords: the recent Adobe hack that affected millions of customer accounts, and another incident that forced Facebook Twitter and other services to reset passwords after a hacker ran off with 2 million passwords.
"We routinely monitor for irregular activity, and if such activity is detected, we may sometimes reset passwords of affected [Sony Entertainment Network] accounts to protect consumers and their account information," a Sony representative told Polygon.