OP, I would suspect you have been flagged by some US agency for investigation. In these cases the vendor does not have much leeway in telling you what is going on, as revealing the details of an ongoing investigation is likely forbidden.
What kind of content is in the app? Do you have any user generated content like comments, etc.? Could be a user of yours posted something that flagged your app.
Unlikely, unless OP is lying about doing sketchy stuff. Free-tier services are notoriously abused, and notoriously unsupported. It's more likely that it tripped up some compliance/fraud business rule. Front line support at any large corp will stone wall you in this case.
What kind of content is in the app? Do you have any user generated content like comments, etc.? Could be a user of yours posted something that flagged your app.