Hi @MHandrow,
Changes to the contact owner property can result from integrations, imports, automations, manual updates to the record. To check which one applies, you can always look at the history of a property on a contact record: View a record's property history
Navigate to the contact record in question and find Contact owner in the left sidebar. Hover your mouse over it and click Details. A right sidebar should open with a log of who/what made a change and when.
Best regards!