Secondary node memory arise while balancer doing work

Hi,

We have an sharding instance configured a balancer window for 3:00am - 6:00am(GMT+8) and we observed that while balancer is doing work, all shard's secondary nodes' memory would arise. Such things would not happen on primary nodes. This may be related with the fact that all query requests are on secondary nodes. I have enabled one shard's heapProfiling and uploaded FTDC data and related logs. Please help identify this issue. Thanks!