'Delete Lead from SFDC' always indicates that a deletion was made in SFDC, but it only means that it was initiated by Marketo if the 'Campaign Name' column for that specific entry was populated. If it is not, then that means the deletion was initiated on the SFDC side. If the Campaign Name is populated for that specific entry, and your unable to find the flow step responsible, you may want to log a support ticket with the details of the issue, and they can help identify exactly where it's originating from and why. If the column is empty for that entry, you'll want to consult your SFDC admin to see if there are any workflows/apex triggers which may have resulted in this.