Do you have any DBMS_JOBs running on your database? Do you kill any of those jobs sessions occasionaly?

If you do, the job process will terminate and the message will be written into your event log: "Instance SID has been terminated". Shortly after (depending on your JOB_QUEUE_INTERVAL parameter) Oracle's SNP process will be restarted again and you'll find the following message in your event log: "Initializing PGA for process SNPn in instance prod".

So it doesn't realy mean that your database instance has been terminated, its only one of the background SNPn processes that has been killed.