SQL Server Mirroring kicks in before cluster has time to fail over
Database mirroring questions from TechEd: failovers and partner timeouts


  ... when the cluster is failing over and the principal instance is
  unavailable for 60-90 seconds, the other instances in the mirroring
  session will time-out after ten seconds and initiate a mirroring
  failover... The solution to the problem is to change the partner time-out value - a facility that isn't well known. To change the partner time-out value for a mirroring session, use the following code:

ALTER DATABASE mydatabase SET PARTNER TIMEOUT 90;