How are database backups conducted using SQL Server VSS Writer Service We have few servers where backups are running from an unknown source. The physical_device_name in the backupmediafamily table is something like {922DDBA9-0686-46B8-94BC-XXXXXXXXXX}2. 

I was checking randomly and noticed this kind of backup was running on the database. So I stopped SQL Server VSS Writer service and the backup process stopped. It prompted me to think that backups are being also being conducted by this service. I am still unaware on how Windows/SQL decides to trigger these backup at particular times only since the SQL Server VSS Writer service is running 24*7.

I have also seen cases on few servers where SQL Server VSS Writer is running but those backups are not happening at database level.

Can someone please help me in understanding (identifying) what is causing SQL Server to trigger such backups as I want to stop them across entire organisation.
You will need to identify the Backup strategy which would be using the VSS Writers for taking backups/Restore.

In our environment we had Netbackup and HPDP (Data Protector). While for Netbackup the location would be the master media location ,for HPDP it used to be similar to what you have {922DDBA9-0686-46B8-94BC-XXXXXXXXXX}.

HPDP uses VSS Writers for the backups and it is what was in our case. It can differ in your situation but there is some backup agent which is takin it.