This Knowledgebase article gives detail about the 'Auto Switchover Started' event.
An Autoswitchover is triggered if the switchover action was selected in the Neverfail Application Module Configuration. The switchover can be selected as an action if the current parameters have a certain value for a certain period of time. The automatic switchover can be set as a recovery action for the first, second or third failure.
If the configure network monitoring is enabled, the Autoswitchover can be also triggered when the principal (public) network connection is lost.