I don't know... look at the bug report and the fix [1]. This does look like a bug in WiredTiger. It preallocates all of its cache, so it doesn't look like raw capacity problems. It looks like the internal MongoD issues pushed the problems downstream to the individual MongoS instances.
Comment from the author:
> We looked for an OOM in our logs but couldn't find it. Also, the log lines are from mongos nodes. Data nodes continues to run with severly reduced throughput.
Comment from the author:
> We looked for an OOM in our logs but couldn't find it. Also, the log lines are from mongos nodes. Data nodes continues to run with severly reduced throughput.
[1] https://jira.mongodb.org/browse/WT-2924