Load balancing between Videobridges

Dear All,
We need some helps to figure out the problem of load balancing in our real case as the following. We observe that there are 2 failed videobridges (having 0 in CPU activities, on the left in the below image), we still had other 2, which seem to be operational (on the right side), but we see that the traffic wasn.t loaded equally between the two healthy servers. The rightmost server had more load than the third from the left. Would you please give me some hints to understand what happened at that moment?
Many thanks