Hi,
At first you should check the logger of the PLC to identify the reason for the service boot.
Common reasons related to custom application code are cycle time violation and page faults, but also other critical system events like missing monitored hardware can lead to service mode.
Here in the community, the’re already some helpful posts about how to diagnose service mode, especially if it’s a page fault I suggest first to read this post: Troubleshooting Page Faults
Best regards!