When combine: true is not defined in any extensions yui.yml file it tries to download YUI3 from external Yahoo CDNs.
Steps to reproduce:
1. Configure a server to use HTTPS as a protocol,
2. From PlatformUIBundle yui.yml file remove the line with combine: true,
3. Create a PlatformUI extension bundle with an yui.yml file config, that adds new modules or extending existing ones,
4. Clear cache and regenerate the assets,
5. When loading PlatformUI app notice that YUI3 files are loaded from CDN servers.
Instead of creating new extensions you might want to include eZ Studio bundle or Content on the Fly bundle.