wsrep_sst_auth user permissions
It seems you are using Xtrabackup-v2 method for SST.

So if you are using that, you should set RELOAD, LOCK TABLES, REPLICATION CLIENT and SUPER privileges to work correctly.

If you do not want set directly user pass in my.cnf you cant start mysql server with this command:

service mysql start --wsrep-sst-auth=username:password