When DNS on your primary DNS server stops responding what prevents all your clients from using the other DNS servers in the list?

Just had a "minor" outage because the 1st DNS server in our list went down but was still pingable.  DNS was not working on it but on ALL of our networked computer it would not continue to use the other DNS servers.  So basically we had a total outage when there should have been plenty of redundancy there to continue.

On the client NICS was the DNS order setup like that? DHCP servers NICS?

You have two parts of the failover setup the DNS forwarder/ conditional forwarder record setup and the client side NIC settings. What can you share in regards to this?

All the clients are set via DHCP and most servers are static with the same DNS settings.

DNS Servers:
AD/DNS1
AD/DNS2
AD/DNS3
AD/DNS4

All the DNS servers are using root hints the problem was that AD/DNS1 was still pingable but DNS was not functioning.  So NSLOOKUP was returning (*** Request to UnKnown timed-out) and stopping there.

This is easily recreatable.  
Go to AD/DNS1 stop the "DNS Server" service.
On client do "ipconfig /flushdns"
On client use NSLOOKUP for any valid host name.

You get a result like this:

nslookup <validhost>
DNS request timed out.
    timeout was 2 seconds.
Server:  UnKnown
Address:  <AD/DNS1>

DNS request timed out.
    timeout was 2 seconds.
DNS request timed out.
    timeout was 2 seconds.
*** Request to UnKnown timed-out

It never switches over to any of the alternate DNS servers.  Made for an action packed few hours while I got it fixed.  Whats interesting is that if you shutdown the server "not pingable" this does not happen.  NSLOOKUP goes on to the other DNS servers like you would expect.

to prevent this from happening again and increase the success of your failover take half of your machines and move DNS 3 or 4 as the primary that way you have half as DNS 1 2 3 4 and the other half as DNS 4 3 2 1. If the server was pingable, just hung as a DNS service it is possible that it did not trigger the failover. did you try setting up conditional forwarders on the DNS servers? A conditional forwarder is a DNS server on a network that forwards DNS queries according to the DNS domain name in the query. For example, you can configure a DNS server to forward all the queries that it receives for names ending with corp.contoso.com to the IP address of a specific DNS server or to the IP addresses of multiple DNS servers.