Lead / Case ownership changed automatically

If you encounter this issue or a similar behavior, there are a few things you might want to check. 

• Record ownership could be changed by Assignment Rules, make sure to check these and compare them against the affected records.
  • Assignment Rules can be found here:
  • Salesforce Classic:
    • Setup | Customize | Leads | Lead Assignment Rules
    • Setup | Customize | Cases | Case Assignment Rules
  • Lightning Experience:
    • Setup | Home | Feature Settings | Marketing | Lead Assignment Rules
    • Setup | Home | Feature Settings | Service | Case Assignment Rules
• Salesforce will evaluate all assignment rules and assign accordingly. If no assignment rule is found, Salesforce will assign this lead to the default owner.
  • You can find the default Owner by following the steps below:
  • Salesforce Classic:
    • Setup | Customize | Leads | Lead Settings
    • Setup | Customize | Cases | Support Settings
  • Lightning Experience
    • Setup | Home | Feature Settings | Marketing | Lead Settings
    • Setup | Home | Feature Settings | Service | Support Settings
• If the change seems to have been made by a user, who wasn't even logged in at the time of the change, you might want to check that user's login history. Very often, third party apps (i.e Appexchange apps) have a default user. Any changes made by the app will be shown as made by this default user. Viewing the user's login history can help you identify if this user is logging in through a third party app, which eventually might be responsible for the ownership change. 
• Case / Lead ownership could also be changed from automation such as Apex Triggers, Workflow Rules, Flows or Process created in Process Builder. An automation could be triggered when the lead is created or updated that will perform an action of changing the owner of the record.

See also:
Managing Assignment Rules
Define Default Settings for Lead Creation