Outlook tries to connect to a dumb CAS Server

Hi team,

I set up a lab environment with a Single Ex server(lets say EX1.domain.com) with both CAS/MBX collocated. No internet, just internal testing. Created mail.domain.com and autodiscover.domain.com and pointed out to EX1 server. All connections are smoothly going.

Then I decided to add another two CAS servers (EX2, EX3) to the organization and did nothing more than installing, didn't configure array even.

Now, I'm seeing that whenever I disconnect my NIC on EX2 and EX3, some of the clients(Outlook) disconnects and some keeps working. What I'm confused is even after NOT doing anything other than installing, why is my client connecting to the new CAS Server(s)?

Thank you.
Cheers!