network directory security credentials password keeps gett..

i've got a virtual directory that accesses a folder on another server,
so i've set up network directory security credentials using an AD user
account with no password expiration. after a week or so, i'll get a
complaint from a user saying the app is failing. when i look at the
password used to authenticate the IIS virtual directory, the field is
blank (thought the userName is still populated).

what would cause IIS to throw out the password?

tks