You can investigate by navigating to a contact record. On a contact record, click View property history (in the About this contact section in the left sidebar). You should now see a property history for this contact and be able to search for "contact owner". HubSpot should show you when and how the contact owner was changed. For example:
Hope this helps!
Karsten Köhler HubSpot Freelancer | RevOps & CRM Consultant | Community Hall of Famer