Enterprise Vault using guest account

I'm using EV10 SP3, Outlook 2013 and Exch 2010.  We use vault cache and virtual vault on Win8.1, EV server is 2008 R2. 

We are rolling out a new SOE.  I have auditing set up on security logs on EV servers.  I am seeing some interesting behaviour.  Some users have had logon issues, and in the event log I can see the failed logon events.  After a few of these, I get the event saying the local Windows guest account on the server is locked out.  The guest account is disabled by default. 

My question is, what is the releationship between users connecting to EV to synch their vault cache and the EV server's guest Windows account? 

Thanks.