searched kb and pr, couldn't find particular things...
also checked chassid before and after time that card went off. Some logs on chassisd says the following.... 8 and 7 are not correct that Junos generated on the device because no one touched the device.
Packet-based, or stateless, packet processing treats packets discretely. Each packet is assessed individually for treatment. For the distributed processing architecture of the services gateway, some packet-based processing, such as traffic shaping, occurs on the NPU.
To know the FRU offline reason we might need the logs from messages and chassis files. So, it will be best if you open a JTAC case for a thorough analysis of this.
The Network Processing Unit (NPU) and the Network Processing session
An NPU is a processor that runs on an I/O card (IOC) and processes packets discretely. When a flow is created, subsequent packets of the flow are matched to the session on the NPU. The NPU handles additional processing such as TCP sequence check, time-to-live (TTL) processing, and Layer 2 header translation. An NPU improves performance in that extra packet forwarding between a session-SPU and a hash-SPU is avoided. The SRX4600 Services Gateway implements an NPU.
You can refer the below link which has details about NPU and where all it could be useful and what functions it does: