If the static bubble HTML is not specified (this is possible for events using OnBeforeEventRender) the event content will be loaded from the server side using the backend controller specified in the main control (e.g. Calendar).
In order to generate the bubble HTML you need to handle OnEventBubble event on the main control:
protected override void OnEventBubble(EventBubbleArgs e)
{
e.BubbleHtml = "This is an event bubble for id: " + e.Id;
}