Hello @Syed_Ahsan_Hasan_Khan ,
“Received signal”,“attr”:{“signal”:15,“error”:“Terminated”}}
As discussed in this thread. Signal 15 also known as SIGTERM is sent to terminate a program, and is relatively normal behaviour. This indicates system has delivered a SIGTERM to the process. This is usually at the request of some other process but could also be sent by your process to itself.
Without knowing any details about your deployment, one avenue you can investigate is checking the default service timeout, maybe it is set lower than the startup time required due to unclean shutdown of your mongod process. You might need a larger timeout similar to this discussion. Also you can refer to this thread as this might have some helpful pointers on how to change systemd service timeout value .
Hope this helps! 
Regards,
Tarun