Thanks Elliot for the reply.
We have no active assignment rules in Salesforce.
Also, I see that the username is MarketoUser in backend of Marketo under Salesforce integration. When I evaluated a few leads that came in, none are affected by a change Owner flow step, which leads me to believe it's after the lead is in SFDC does the owner change.
Looking at the leads, I notice that if Marketing sent an email to the person, it changes the lead owner to Marketing instead of MarketoUser. A strange but oddly coincidental feature.