When you did your CAS array you gave it a name (say array.domain.com) to what IP this name is resolvable to?
Since your CAS servers and your Mailbox servers are on the same servers then you cannot use Windows Load Balancing for your CAS so how did you load balance your CAS servers ? Do you have a hardware load balancer ?
Did you change the configuration of your databases to use the cas array name as their RpcClientAccessServer ? you can do this by running get-mailboxdatabase | set-mailboxdatabase -RpcClientAccessServer array.domain.com
In your scenario you basically have 2 options
option 1
1. Get a hardware load balancer to load balance your CAS servers
2. create in your dns a record where array.domain.com points to the NLB IP of your CAS server
3. set the RpcClientAccessServer of your mailbox databases to array.domain.com
Option 2
1. Create in your DNS a record where array.domain.com point to the IP of one of your CAS servers with a low ttl
2. set the RpcClientAccessServer of your mailbox databases to array.domain.com
3. when you have a failure change in the DNS the IP of array.domain.com to the IP of the CAS server not failing