Database mirroring - link failure between principal and witness
The problem is that the principal became disconnected from the witness server before it went offline. In this case, the witness cannot agree that the principal is offline because it was disconnected while the principal was still connected. As far as the witness knows, the principal may still be up.

In this scenario, it would require that you force service on the mirror to bring it online using ALTER DATABASE ... SET PARTNER FORCE_SERVICE_ALLOW_DATA_LOSS.