Why are users being unenrolled for no apparent reason?
Please check the following: • The longtimenosee variable in Site Administration > Server > Cleanup (or Administration > Configuration > Variables in Moodle 1.6) which specifies the time limit for which, if users (teachers as well as students) haven’t logged in, they are unenrolled from courses. • The Enrolment duration in the Course settings which unenrols students after the specified time has elapsed.