You see something like the following error message in the Message Logs of the backup:
backup.c:666 Network send error to SD. ERR=An existing connection was forcibly closed by the remote host.
This can happen when the Storage Daemon shuts down due to being killed by a process which looks for out-of-space issues.
It can also be due to a network connectivity problem in which the TCP stack on the CFA eventually closes the connection with a RESET.