I am not quite sure about the number of connections, but it could have something to do with default connection pool settings for Mongoose. The pool size, by default, is probably 5. Check the documentation here:
http://mongoosejs.com/docs/connections.html
Also, on the additional members, you are using MongoHQ Elastic Deployments. These are multi-member replica sets (giving you high availability in the event that the primary database has an issue). So, make sure that you check the documentation for how Mongoose handles replica set configurations. That way, your app will stay online if a replica set has a step down.
Hope this helps!