If the Chrome browser zoom is different from 100%, it is not possible to reorder the RadGrid columns. The problem can be reproduced at https://demos.telerik.com/aspnet-ajax/grid/examples/columns-rows/columns/column-row-resize-reorder/defaultcs.aspx .
Closing as duplicate of the following item (which will be fixed in R2 2018, btw, and you can use the workaround in the meantime) https://feedback.telerik.com/Project/108/Feedback/Details/52656-radgrid-items-drag-drop-functionality-does-not-work-correctly-when-chrome-is-zoom
Not sure whether a like or dislike vote counts towards fixing this bug...?
This is causing problems for our clients and would be great if it was addressed soon.