In versions before 4.3, the X-UA-Compatible header was set to IE=9 in order to force rendering engine compatibility mode. In 4.3, this header was removed because it caused issues with the some new UX stuff.
On most configurations, this is OK because MSIE will, by default, use its latest available rendering engine. But in cases where MSIE settings are set to an incompatible compatibility version (e.g IE=8), users get the compatibility issue page. This is especially annoying in places where compatibility settings are enforced by company policy.