It is bundling and or minimizing the CSS. MVC does it automatically in MVC. If you don't want to do it you can shut it off.
In your development environment you can set debug true
<system.web>
<compilation debug="true" />
</system.web>
or in the bundle.config you can set
BundleTable.EnableOptimizations = true;
which will render it inert. To do this in production and you don't want to debug, then just use
BundleTable.EnableOptimizations = true;
Resource:
http://www.asp.net/mvc/overview/performance/bundling-and-minification[
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