I just received the following notice from Sony in regards to the "irregular activity" mentioned in the original article. They encourage subscribers to create a new complex password to be safe.
Sony is preemptively resetting passwords of select PSN users as a precautionary measure after noticing "irregular activity" on some accounts.
In a statement sent to Kotaku, a Sony representative stated, "We routinely monitor for irregular activity, and if such activity is detected, we may sometimes reset passwords of affected SEN accounts as a precautionary measure to protect consumers and their account information."
Users who would like to change their password themselves can do so through the Sony Entertainment Network website provided they have the email address and birth date of the account holder. If not, they will have to contact Sony directly.