Simply calling a parent control's event from a child control is called Event bubbling. Handling child controls events through parent control in data bind controls known as event bubbling. Server controls like Datagrid, DataList, and Repeater can have other child controls inside them.Example DataGrid can have combo box inside datagrid. These child control do not raise there events by themselves, rather they pass the event to the container parent (which can be a datagrid, datalist, repeater), which passed to the page as "ItemCommand" event. As the child control send events to parent it is termed as event bubbling.

when you create the button inside the template control writting event handler for all the controls is the tedious task so the parent control event_handler is fired for all the child controls.This process is known as event_bubbling.