Did you happen to recently delete a bunch of inactive or bad records from Marketo? I'm guessing what might've happened is that you removed a bunch of records from Marketo, but left them in SFDC, and something happened to update the sysmod timestamp for those records in SFDC on the 23rd. Marketo sees that and creates the lead in response. In this case, your best option is probably to create a recurring smart campaign that deletes your inactive records on a regular basis.