Hi Scott,
Leads in the Marketo Lead Database do not automatically sync to SFDC. They must be synced to SFDC via the Sync Lead Flow Action. This can be done by a single flow action from the Lead Database, or by a batch or trigger campaign in Marketing Activities.
If you have a Trigger Campaign that is automatically syncing leads to SFDC, you can edit the campaign to exclude the Static List that you are importing to. If you require further assistance with identifying campaigns that are syncing leads to SFDC, I would suggest to submit a Support Case.
Thanks.