When you have a complex control, like DataGrid, writing an event processing routine for each object (cell, button, row, etc.) is quite tedious. The controls can bubble up their event handlers, allowing the main DataGrid event handler to take care of its constituents.

Well, when you have Gridview/Datagrid control showing data. Sometimes you may have complex requirement of editing a record, while editing some columns may have choice input dropdown or radio. Based on that you may need to fill up another subgrid or another column value.Now in such scenarios, you will not directly get that index_changed event in dropdown in a gridview row.Now such events are bubbled with grid event to resolve the complexity and make coding better. On the edit item or item command event you can get the event of sub items and this is called event bubbling.