There are two ways to check your setup. See if database is set to auto shrink (1) and see if you have a Maintenace plan setup that auto shrinks the db (2)
1. Connect to SQL Management Studio
Expand Databases (click + sign next to Databases)
Right click on the B1 database
Select options
Auto shrink is usually set to False as per SAP guidelines
2. Connect to SQL Management Studio
Expand Management
Expand Maintenance Plans
If you have created backups or maintenance you will see a list of existing plans
Right click on the appropriate plan and select MODIFY
From this point on you can review the plan setup and modify if necessary
Hope this helps.