account lockout problem

We have a Win2k domain w 3 DC’s one of which runs Exchange. Our remote users using OWA are having a problem with their account getting locked out. The lockout policy is to lockout afer 3 failed logon attempts, but the users in question swear they aren’t doing it (ok, I know – users) Anyway, is there anything else that can cause a lockout. We are trying to decide if we are dealing with unauthorized access attempts, or moronic users or some other phenomenon. If you don’t know of any other cause of lockout, can youu suggest a trouble-shooting procedure? Wre are auditing account logon success and failure, but no failures are being logged. Help!