We'v set up sharded cluster with replica set for each shard in our production ENV. Last night we encountered a issue while connecting to the cluster and issuing building index command through mongo shell , by connecting to the mongos instance , not directly to specific shard.

The issue is : once starting building the index, connections created from mongos to this shard increases rapidly and 'too many connections' errors show up in the primary shard's log file very soon. 

From the three mongos' log, all the connections are initiated from mongos. We have googled and find related issue link as : https://jira.mongodb.org/browse/SERVER-28822 But there is no trigger conditions. And the same time, I tried to reproduce the question in test ENV ,but not occurred again. So, please help.

The attachment is the log details while mongod encountered the error.