F5 5520-V Random shutdown

Hi,

We have two F5 5520-V's which are clustered, in the active/standy configuration. Recently, the active device powered it's self off and required a engineer to attend site and manually restart the box. There was power going to the device and both PSU switches were in the ON position. There are no ominous error messages any where in the logs that I have looked at. Has any one else experienced this type of issue or know what could of caused it?