Check your programs that have anything to do with Sync to SFDC... whether it's ownership changes, adding to campaigns, assigning through routing rules, etc.
I think by default Marketo will use around 7k API calls per day, but your campaigns can blow that number up if you're not careful.
I recommend using the Campaign Inspector and search for SFDC to see if you can find a culprit. If not, Marketo support should be able to help.