Ping Failing from collector to device and back?

Yes I think the collector are overloaded as all of them for the datacenter implied here are over the Load Balancing settings which would mean there is no more balancing anymore !!!

Auto balanced collector group (ABCG) does not mean load balancing. It is overload redistribution. 

Example: Two collectors with rebalance threshold of 10,000. One collector has 9,999, the other has 5. No rebalancing happens in this case.

When/if the first collector gets more than 10,000, the device with the highest number of instances will be reassigned to a different collector. If that brings the count back below 10,000, rebalancing stops. 

Example: Two collectors with rebalance threshold of 10,000. One collector has 10,001, the other has 5. Largest device on the first collector has 200 instances. Rebalancing happens. The first collector ends up with 9,801, the other has 205.

ABGC != Load Balancing.