I assume that changing the columns text is done similar to headerItem["ColumnName"].Text, if that is the case it is expected to disable sorting.
Here is a forum post describing how to change the Column headertext in the code behind: Changing RadGrid Column Header Text when Sorting is Enabled
When sorting is enabled, the cell content will contain a Button in it. By changing the cell.Text (headerItem["ColumnName"].Text) overwrites the button with a literal and thus loosing the sorting option. Instead, you will need to access the control in the cell, and set that control's Text.
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