What causes repeated linux NFS client side RPC credential refreshing?

I encountered this exact issue with NFS. The problem in my case was caused by actimeo being set too low. While you may not be using this exact setting, there is a whole family of settings that can cause havoc: acregmin, acregmax, acdirmin, and acdirmax. Essentially what happens is that the system is caching the file attributes from the NFS. These settings affect how long the file attributes are kept before refreshing from NFS. On a system with heavy usage, these refreshes become painfully obvious.

Another problematic setting is noac. If you use this, you guarantee that any writes will be immediately available to all other clients. However, writes will wait until finished writing to the remote rather than using write-behind. This can bring a system to its knees if it frequently writes to NFS.

This is an interesting blog article where they talk about the different options and their effect on NFS performance. You could also look at the man page for NFS for more guidance. Unfortunately, authrefrsh can be a bit of a red herring and my issue may be totally unrelated, albeit with similar symptoms.