Hello Kim,
If you look in the Activity Log page for the lead in question and search for keywords such as "owner" or "lead owner", you should be able to find the activity that shows the lead owner change from and to. On the same line where this activity shows, see if there is anything in the "Campaign Name" column which would indicate a Marketo campaign or single flow action that made this change. If nothing shows in the "Campaign Name" column, double click on the activity that shows the lead owner change. By double clicking it opens to the "Detail" page. In the "Detail" page, look for "Source" to see what it shows. It may see LeadDB, API or even SFDC which would indicate what the source of this change was.
If the change is via a MKTO campaign that shows in the "Campaign Name" column ( noted above ), please visit the campaign in question and adjust the flow steps so the lead will be assigned as you wish.
Regards,
Romer