One thing to keep in mind is that if the visibility rules change, Marketo won't actually see those people until a field that Marketo can see changes. Marketo looks at the "sysmodtime" stamp on a record. This indicates the last time a record was modified. If the record was changed since the last time the sync ran, then Marketo updates the record on the Marketo and SFDC side with the newest information.
There have been cases where the visibility rules changed, but the new records that Marketo could see weren't updated for a considerable period of time. (This could be as long as years.) However, if something eventually changed and updated the sysmodtime, these leads would all sync to Marketo. The change can be anything, but if it's a large number of people, it's frequently a change on the Account object, which then filters down to the Contacts and Leads.
If you're still unsure why a large number of people have synced to Marketo, the first thing to look at is what information changed, since Marketo only syncs people when they are updated with new information.