Each of these emails have come from someone I know. Their accounts at Yahoo have been hacked. In my experience, this happens with Yahoo accounts much more frequently that any other email provider out there.
My advice, secure your account heavily, or dump your Yahoo account and go to another email provider. In terms of security, make sure you have a very strong password. One so strong that you yourself have a hard time remembering it. Make sure you change your password security questions and answers. And, make sure you set up two factor authentication for changing your password.