If you look at the full list, it seems like it's just autocompleting words in the dictionary. Here's some examples:

    if.corp.microsoft.com
    my.corp.microsoft.com
    one.corp.microsoft.com
    year.corp.microsoft.com
    baby.corp.microsoft.com
    pain.corp.microsoft.com
    crisis.corp.microsoft.com
    improvement.corp.microsoft.com
    devil.corp.microsoft.com
    animal.redmond.corp.microsoft.com

I don't any of these some of those are actual subdomains...

Looks more like they're looking for known pieces of domains rather than really parsing the domain as a whole.