With a lack of heapdump for you to analyse, my hypothesis is that your DC2 nodes are taking on traffic (from some client somewhere) but you're just not aware of it. The hints replay is just a side-effect of the nodes getting overloaded.
To rule out my hypothesis in the first instance, my recommendation is to monitor the incoming connections to the nodes in DC2. If you don't have monitoring in place, you could simply run netstat at regular intervals and go from there. Cheers! GOT QUESTIONS? Apache Cassandra experts from the community and DataStax have answers! Share your expertise on https://community.datastax.com/.
