I started to isolate my code - then realized that this problem is easily reproduced.
Take this example:
1. Add AllowSorting="True" to the RadGrid
2. Wrap the RadGrid in a RadAjaxPanel - or wire it to a RadAjaxManager
3. In a normal desktop resolution that should NOT apply the @media CSS rules - Click a Column heading to sort
NOTE: The @media rules are applied to the grid erroneously. I have also had this apply other @media rules on my page - outside of the radgrid and radajax on column sort or grid paging. This problem stops if you remove the Ajax wiring - which then of course makes the page much less usable.
Tested in the latest version of Chrome on Windows 8.1 and using the latest 2015.2 release