Unregular load and merges

Elastic Stack Elasticsearch
1

We have 6 es nodes (cc2 8xlarge). Right now we're indexing about 5 billions
documents to 87 different indexes each with one shard and replication 1.
Documents are regularly distributed so there are no hotspots. Documents on
each index/node grows with same speed but after 3 billions documents are
written I see a strange think.
One of the nodes has load 30 and the others under 10 and it seems that the
same node doing the merges much more often than the others.

here is index settings:
{
"index" : {
"compound_on_flush" : "false",
"creation_date" : "1426854580131",
"number_of_replicas" : "1",
"compound_format" : "false",
"uuid" : "XTUf5SLKSqCK2fWtkqkn1g",
"number_of_shards" : "1",
"refresh_interval" : "-1",
"version" : {
"created" : "1040399"
}
}
}

Do you have any idea why it's happening?

https://lh6.googleusercontent.com/-WCkIvwBF2HY/VRAwQ4Xy3bI/AAAAAAAAABY/KM3UTktAKkM/s1600/FireShot%2BCapture%2B-%2Bkopf%2B-%2Bhttp___solr-perf-test2.us-w2.aws.ccl_9200__plugin_kopf_%23!_nodes.png

https://lh5.googleusercontent.com/-H4cUVNeyZCM/VRAwJlCCfRI/AAAAAAAAABQ/4Na4ZE6UfJo/s1600/FireShot%2BCapture%2B-%2BElasticsearch%2B-%2Bes-perf-test%2BDas_%2B-%2Bhttp___graphitus.us-w2.aws.ccl_dashboard.html.png

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2

about 12:00PM I've changed the index settings to

"index" : {
"refresh_interval" : "-1",
"compound_format" : "false",
"compound_on_flush" : "false"
}

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