If I remember correctly telerik does not render nor adds all the data to viewstate. So you will need to reset the DataSource every time the code behind is called in this case paging. You can avoid the effort in identifying each action by setting the datasource on onneeddatasource event.
This is mentioned on Telerik Grid common Mistakes: http://www.telerik.com/help/aspnet-ajax/grid-most-common-mistakes.html
- Missing or not using NeedDataSource event The NeedDataSource event helps developers easily control scenarios like paging, sorting, and grouping, with Telerik RadGrid . Using these types of PostBack events, which RadGrid fires, can lead to a change in the Items collections of each GridTableView in a Telerik RadGrid .