investigate exactly what crontab is running those orphan tasks (from /var/spool/cron/crontab?) Looking at "ps -aux|grep cron" should tell you what cron is running as.

Sometimes an install of an application (lets say mailman) creates a user and installs a crontab entry, then you ditch the mailman account and its home directory manually keeping the crontab in place (though this would be an old or bad version of cron that would allow for such thing)

You could also look at this syntax to make sure ...