SQL Server Mirroring and automatic failover - why don't all DBs failover to the mirror?
The issue is that there is a separate ping for each database, not one for all. So some are successful and some fail. If you need all of the databases to fail over, I outlined a process (using service broker and event notifications) to detect a failover and make sure all mirrored databases fail over in my book Pro SQL Server 2008 Mirroring.

A lot of this information is also located on my blog here Database Mirroring Automation and here: How do I automatically respond to mirrored database failovers?