This is caused by some device/application/script trying to connect to your configured NDMP port (default 10000) using the login of "root".  Once this was explained to me by Isilon support i did a packet capture to see whom the offender was. In my case it was a Netapp Oncommand application that was configured to do auto discovery in the same range as my isilons (a common storage network). 
On a brand new 7.2.1 install where ndmp is currently working (backes from isilon) I get the below error repeated over and over constantly.   2016-04-05T05:44:26-04:00 <3.3> node-2(id2) isi_ndmp_d[37692]: CRIT:NDMP security.c:377:securityAuthenticateClient Failed to get password of root 2016-04-05T06:03:56-04:00 <3.3> node-2(id2) isi_ndmp_d[38841]: CRIT:NDMP security.c:377:securityAuthenticateClient Failed to get password of root 2016-04-05T06:23:26-04:00 <3.3> node-2(id2) isi_ndmp_d[40113]: CRIT:NDMP security.c:377:securityAuthenticateClient Failed to get password of root   The NDMP user is working as the backup DMA is using it successfully. Additionally i can login as root. This error does not seem to have any ill effects, but it spams my log files constantly. 
New install on 7.2.1, NDMP logs filled with error.