Each click on an elements in content apps leads to requests to the version store. This can be expensive especially for big installations. I have not tracked it down but my guess would be that it is a availability test for an action which checks wether the node has versions or not.
To verify this one can enable derby logging like described here: http://magnolia-experts.com/monitoring-queries-to-the-derby-database/
Attention: jackrabbit caches nodes in memory (16MB bei default) so with a small installation like the demo project the queries will be only executed during the first access and not by follow up browsing.