Multi-site SQL Server Cluster issue We have a 12 node SQL Server FCI (6 nodes in our primary datacenter and the other 6 nodes at a DR site) which has been working well until an access control list (ACL) was applied to a network switch. 

When the ACL was applied, the primary cluster was not able to access/communicate with the 6 DR nodes. As a result, disks on the primary DB nodes started to dismount and the cluster stopped functioning. 

Any ideas as to why this is happening?
The issue that occurred is quorum was lost - This is a good primer on how the cluster voting works and should give you some insight into how to design a cluster to meet certain disaster scenarios.

Since you have an even number of nodes on each "side" and connectivity was lost between the "sides", the cluster didn't know who should actually own and run the resources - this is something called split brain.

Generally, we have some sort of tie breaker involved. This could be as simple as a fileshare or disk. It could be as convoluted as cluster property settings or even sites with preferences. Generally, though, for most scenarios it can be pretty basic and is discussed in the post I linked above.

If you're looking to not have this happen in the future then I'd suggest, depending on version of Windows Server, think about the scenarios you're wanting to defend against and setting the cluster to have the resources or cluster level settings needed to fend those potential issues off. A cluster does not insulate you from every possible issue but does lend a great deal of high availability to most scenarios.