Utazo: I don't know. I set up my account over a year ago, and it's never deleted itself. Nobody else has reported that problem here.
Do you use Titanium Backup or some other kind of backup software (perhaps maybe even something built-into Android 4.0)? Is it possible that such software is "restoring" your earlier account information and thereby deleting your Hotmail account?
Let me know if you figure it out.