|If you encounter this issue or a similar behavior, there are a few things you might want to check.
- Lead ownership could be changed by Lead Assignment Rules, make sure to check these and compare them against the affected records.
- If no Lead Assignment Rule has been created, take into account that Salesforce has a default Lead Assignment Rule. Under Setup/Customize/Leads/Lead Settings, you will find the default lead owner. If a user edits a lead record and checks the "assign using active assignment rules" checkbox, Salesforce will evaluate all assignment rules and assign accordingly. If no assignment rule is found, Salesforce will assign this lead to the default lead owner.
- 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.