If I were to guess I'd say that your app is hitting a hardware/software load-balancer or DNS VIP or DNS round-robin to get to the cluster and so it's only routing traffic to the old nodes.
If this is the case, I would recommend you use the built-in load-balancing policy on the C* drivers instead. I've explained the reasons in detail on https://community.datastax.com/questions/7702/ and https://community.datastax.com/questions/4390/. Cheers! >
