You don't necessarily want to announce what suspicious activities are being looked for. That just lets criminals know what activities to avoid.
I know, but I wouldn't also want to have my account frozen by myself.
If the purpose of such freezing is really to protect the account owner, then maybe the triggers of such system could be configurable by the user himself. Not allowing the user to configure what could trigger his account freezing gives the impression that such mechanism is not there only to protect the users...