Microsoft has released an important update for customers who have deployed Cumulative Update 6 for Exchange Server 2013 in a co-existence environment with Exchange Server 2007.
Support article KB2997209 is titled Exchange Server 2013 databases unexpectedly fail over in a co-existence environment with Exchange Server 2007, which is a pretty good description of the issue that customers are experiencing.
The problem occurs when an Exchange 2007 mailbox user tries to access a delegated mailbox that is hosted on Exchange 2013 CU6. The Exchange 2013 CU6 server’s attempt to authenticate the connection causes the Store process to crash, which in a database availability group will result in a database failover to another DAG member.
The available hotfix can be obtained by contacting Microsoft Support. More details are available here.