Mongos distribute too much query to one MongoD - mongodb

Question

We have a mongodb with 2 shardings, each of the shardings has those servers:
Shard 1: Master, running MongoD and Config server
Shard 1-s1: Slave, running MongoD and MongoS server
Shard 1-s2: Slave, running MongoD and MongoS and Arbiter server
Shard 2: Master, running MongoD and Config Server
Shard 2-s1: Slave, running MongoD and Config and MongoS server
Shard 2-s2: Slave, running MongoD and MongoS and Arbiter server
But the mongodb allways failed in recent days, after days of search, i find out that the MongoD runs at Shard 1(Master) always going down after reviced too many connections, other MongoD don't have this problem.
When the S1Master's MongoD running with too many connections for about 2 hours, the 4 Mongos Server will shut down one by one. Here is the Mongos's error log(10.81.4.72:7100 runs MongoD):
Tue Aug 20 20:01:52 [conn8526] DBClientCursor::init call() failed
Tue Aug 20 20:01:52 [conn3897] ns: user.dev could not initialize cursor across all shards because : stale config detected for ns: user.
dev ParallelCursor::_init # s01/10.36.31.36:7100,10.42.50.24:7100,10.81.4.72:7100 attempt: 0
Tue Aug 20 20:01:52 [conn744] ns: user.dev could not initialize cursor across all shards because : stale config detected for ns: user.d
ev ParallelCursor::_init # s01/10.36.31.36:7100,10.42.50.24:7100,10.81.4.72:7100 attempt: 0
I don't know why this mongod revieved so many connections, the chunks shows the sharding works well.