readPreference:nearest causing connect / end sprawl

I'm using readPreference:nearest with my replica set, and I noticed that setting this option causes my logs to grow (a lot over time). I checked out my logs, and I see a bunch of rows that repeat (causing the growth):

connection accepted from IP
end connection IP

This doesn't happen when I set readPreference to anything else. I don't want to incur the overhead of connect/reconnect, so I'm wondering if this is a problem, or it it's normal behavior?

Here's my client code, using node native driver:

var server = 'mongodb://'+dbh[0]+','+dbh[1]+','+dbh[2]+'/0?w=3&readPreference=nearest';
  var options = {
    db: {},
    server: {},
    replSet: {
      rs_name: 'rs0',
      strategy: 'ping',
      secondaryAcceptableLatencyMS: 50,
    },
    mongos: {}
  };
  _m.MongoClient.connect(server, options, function(err, db){