Some time ago a customer reported an issue with their Exchange 2010 databases being failed over at random times during the day. They had Exchange 2010 DAG environment with 4 servers on the primary and two on the secondary site. Databases on the primary site would randomly crash and failover to another Exchange 2010 server. When inspecting the logs I found this event in the Application log:
Source: MSExchange AD Access
Event ID: 2389
Description: Process STORE.EXE (PID=2668). A request to directory server somedc03.somedomain.hr did not return a result within 30 seconds and is being abandoned. The search will be retried if possible.
Just after that, the databases would crash. This is a screenshot from the Application log: