Why MongoDB collections are dropped automatically?
I have gone through your error log and find out the MongoDB Bug affected version error , which are able to see in your error log such as:


  "mongodb failed to create WiredTiger bulk cursor: Device or resource
  busy"


As per MongoDB jira documentation here as it is Server Bug having affected version was 2.8.0-rc0.

The Bug fix version of MongoDB is 2.8.0-rc1.

And the second Bug error, which is


  2018-08-28T12:44:46.138+0530 I CONTROL [signalProcessingThread] got
  signal 1 (Hangup), will terminate after current cmd ends


As jira documentation here It's affected Bug in MongoDB version 3.0.12. 

As per @RamonFernandez from jira documentation, seems like mongod is receiving the SIGHUP signal and terminating, which is expected behavior. You need to check who or what is sending this signal.