I noticed that our site running b2evolution 4.1.7 open the admin panel in IE7 standards mode when browsing it with Internet Explorer 9.

I was able to fix this by modifying the header information for the admin skin and adding the tag:
<meta http-equiv="X-UA-Compatible" content="IE=edge" />

But what could cause this? I tested the demo page for b2evolution with the same browser and it rendered correctly in IE9 standards.