A huge amount of user research went into the original ribbon. However, it looks like they decided that no further research was needed and just started moving everything into the ribbons without any thinking whatsoever.
I haven’t used Excel for any serious work since.
Thankfully, Internet Archive preserved the "Why the new UI" series of posts: https://web.archive.org/web/20080316101025/http://blogs.msdn...
Relevant parts: by Office 2003 they had 250 top-level menu items and 50 toolbars (Part V), and this was becoming unmanageable. In Office 2003 they started collecting info on how people are using Office: About 1.3 billion sessions since shipping Office 2003, over 352 million command bar clicks in Word over the last 90 days (Part VI).
I won't quote the rest because it's a very good series of articles.
Another example is CAD software like Solidworks, but that’s a masterclass in awful UI design all on its own.