I suspect it is hard coded IE versions in Dynamics CRM code
A few of our .Net based sites broke in IE 10 when it was released because Microsoft had hard coded IE version numbers into the .Net framework. The fix was a patch for .Net that added the newer versions into the list of compatible browsers. For the versions that weren't on the list it decided you had an unsupported browser and only allowed basic functionality to work.
I suspect this is the same issue turned up to 11.