We know this effect. It is related to the scroll bar settings changed accordingly while you are resizing a column. This may be a general problem for other grids that allow the user to resize columns and see the effect immediately. For instance, you can reproduce something similar in the Windows Explorer in the file pane in the details view.
In a nutshell, when you shrink a column, the maximum scroll bar value and its current value (the position) are decreased too. This causes the horizontal scrolling on the screen. The MS Header control we use as the iGrid header "thinks" that the mouse pointer has been moved, and this causes the next column resizing event, and so on.
If this behavior annoys you a lot, let us know. Perhaps, we will find a workaround for this.