This behaviour only occurs in Chrome Webbrowser if you select an item in the grid in the bottom area of a list which has of more than 20 elements.
If you start to drag the item, it appears far to low which means that it is sometimes not on the page anymore.
It is even possible to redo this with the telerik demo ( on http://demos.telerik.com/aspnet-ajax/grid/examples/columns-rows/rows/drag-and-drop/defaultcs.aspx)
if you use the pager some times.