Hi Christina,
We had a very similar issue recently where Marketo was making our our API usage for the day, rending our other SFDC plug-ins unusable. Unfortunately, there is no way in your instance to see exaclty what is causing the spikes in usage. I had several calls with Marketo Support and that team even has difficulty pinpointing exacly what it is. They literally went through all of my programs to identify which were using up more calls. Needless to say it was furstrating. One good thing that came of it though is they were able to set an API call limit on our integration, preventing Marketo from eating up our API limit going forward.
I hope this helps!