ASA what triggers an active-standby failover
This command specifies the interface that is monitored by the feature. If that interface fails, then the failover will be triggered. For example:
ASA1(config)# monitor-interface INSIDE ASA1(config)# monitor-interface OUTSIDE
The above configuration causes the ASA to consider itself failed if either the INSIDE or the OUTSIDE interface goes down. It will cede active status to the other ASA.
To understand the behavior of the ASA when such an interface comes back up, take a look at Cisco ASA Active-Standby failover preemption.
To understand what happens when the link between Active/Standby ASAs fails, take a look at ASA active-standby failover preemption.