mongo converting standalone to replica set causes services down

I have standalone node1 already running for a long time. Now I want to convert it to replica set. I followed steps in https://docs.mongodb.org/v2.4/tutorial/convert-standalone-to-replica-set/ to convert standalone to replica set.

After I added a member in mongo client connnected to primary, command line prompt changed from ReplicaSet0:PRIMARY> to ReplicaSet0:SECONDARY>. Then I found my production's services were down.

I check my sentry(an error collection service), and found lots of errors my Ruby code threw:

Moped::Errors::ConnectionFailure: Could not connect to a primary node for replica set #<Moped::Cluster:28315820 @seeds=[<Moped::Node resolved_address="10.128.129.90:27017">, <Moped::Node resolved_address="10.128.130.139:27017">]>

These are my operation and mongo output:

ReplicaSet0:PRIMARY> rs.add("node2")
{ "ok" : 1 }
ReplicaSet0:PRIMARY> rs.conf()
{
  "_id" : "ReplicaSet0",
  "version" : 2,
  "members" : [
    {
      "_id" : 0,
      "host" : "node1:27017"
    },
    {
      "_id" : 1,
      "host" : "node2:27017"
    }
  ]
}
ReplicaSet0:PRIMARY> rs.status()
Thu Oct 22 15:40:13.762 DBClientCursor::init call() failed
Thu Oct 22 15:40:13.763 Error: error doing query: failed at src/mongo/shell/query.js:78
Thu Oct 22 15:40:13.763 trying reconnect to 127.0.0.1:27017
Thu Oct 22 15:40:13.764 reconnect 127.0.0.1:27017 ok
ReplicaSet0:SECONDARY> 

You can see PRIMARY became to SECONDARY. Why did it happen? I think it caused my services down. And How can I avoid it? Please help me.

Update0:

mongo.conf(Yeah, that is all.)

dbpath=/data/mongodb
logpath=/var/log/mongodb/mongodb.log
logappend=true
bind_ip = 0.0.0.0
journal=true
replSet=ReplicaSet0

Update1: rs.status()

ReplicaSet0:SECONDARY> rs.status()
{
  "set" : "ReplicaSet0",
  "date" : ISODate("2015-10-22T07:58:14Z"),
  "myState" : 2,
  "members" : [
    {
      "_id" : 0,
      "name" : "kuankr:27017",
      "health" : 1,
      "state" : 2,
      "stateStr" : "SECONDARY",
      "uptime" : 2463,
      "optime" : Timestamp(1445499598, 19),
      "optimeDate" : ISODate("2015-10-22T07:39:58Z"),
      "self" : true
    },
    {
      "_id" : 1,
      "name" : "mongo-primary:27017",
      "health" : 0,
      "state" : 8,
      "stateStr" : "(not reachable/healthy)",
      "uptime" : 0,
      "optime" : Timestamp(0, 0),
      "optimeDate" : ISODate("1970-01-01T00:00:00Z"),
      "lastHeartbeat" : ISODate("2015-10-22T07:58:13Z"),
      "lastHeartbeatRecv" : ISODate("1970-01-01T00:00:00Z"),
      "pingMs" : 0
    }
  ],
  "ok" : 1
}

pick some relevant rows from mongodb.log

34325 Thu Oct 22 15:39:52.139 [conn397] replSet replSetReconfig config object parses ok, 2 members specified 
34326 Thu Oct 22 15:39:54.599 [conn397] replSet replSetReconfig [2]
34327 Thu Oct 22 15:39:54.599 [conn397] replSet info saving a newer config version to local.system.replset
34328 Thu Oct 22 15:39:54.607 [conn397] replSet saveConfigLocally done 
34329 Thu Oct 22 15:39:54.607 [conn397] replSet info : additive change to configuration
34330 Thu Oct 22 15:39:54.607 [conn397] replSet replSetReconfig new config saved locally
34331 Thu Oct 22 15:39:54.607 [conn397] command admin.$cmd command: { replSetReconfig: { _id: "ReplicaSet0", version: 2, members: [ { _id: 0, host: "kuankr:27017" }, { _id:       1.0, host: "mongo-primary" } ] } } ntoreturn:1 keyUpdates:0 locks(micros) W:8249 reslen:37 2467ms
34332 Thu Oct 22 15:39:54.612 [rsHealthPoll] replSet member mongo-primary:27017 is up
34333 Thu Oct 22 15:39:54.612 [rsMgr] replSet total number of votes is even - add arbiter or give one member an extra vote
34334 Thu Oct 22 15:40:08.610 [rsHealthPoll] DBClientCursor::init call() failed
34335 Thu Oct 22 15:40:08.750 [rsHealthPoll] replSet info mongo-primary:27017 is down (or slow to respond):
34336 Thu Oct 22 15:40:08.750 [rsHealthPoll] replSet member mongo-primary:27017 is now in state DOWN
34337 Thu Oct 22 15:40:08.750 [rsMgr] can't see a majority of the set, relinquishing primary
34338 Thu Oct 22 15:40:08.750 [rsMgr] replSet relinquishing primary state
34339 Thu Oct 22 15:40:08.750 [rsMgr] replSet SECONDARY
34340 Thu Oct 22 15:40:08.750 [rsMgr] replSet closing client sockets after relinquishing primary
34341 Thu Oct 22 15:40:08.751 [conn4] end connection 10.128.132.214:47738 (61 connections now open) * 
34402 Thu Oct 22 15:40:08.755 [conn385] end connection 127.0.0.1:35975 (1 connection now open)
34414 Thu Oct 22 15:40:15.895 [rsMgr] replSet info electSelf 0
34415 Thu Oct 22 15:40:15.896 [rsMgr] replSet couldn't elect self, only received 1 votes
34425 Thu Oct 22 15:40:21.897 [rsMgr] replSet info electSelf 0
34426 Thu Oct 22 15:40:21.897 [rsMgr] replSet couldn't elect self, only received 1 votes
34465 Thu Oct 22 15:40:35.897 [rsHealthPoll] DBClientCursor::init call() failed
34466 Thu Oct 22 15:40:35.898 [rsHealthPoll] replSet info mongo-primary:27017 is down (or slow to respond):
34467 Thu Oct 22 15:40:35.898 [rsMgr] replSet can't see a majority, will not try to elect self
34503 Thu Oct 22 15:40:43.899 [rsHealthPoll] replSet member mongo-primary:27017 is up
34504 Thu Oct 22 15:40:43.899 [rsMgr] replSet info electSelf 0
34505 Thu Oct 22 15:40:43.900 [rsMgr] replSet couldn't elect self, only received 1 votes