Microsoft pointed abc.microsoft.com to GitHub pages through dns. That GitHub repo name was deleted; & eventually available back for anybody to get it. New user gets that repo name. Now new user publishes his content on that repo. Now if you type abc.microsoft.com you will see new user's content.
TIL why aka.ms/whatever is used redirects instead of having the redirect service be on a *.microsoft.com domain. Besides being shorter, I assume that this greatly reduces the risk for subdomain takeover as aka.ms shouldn’t have any associated cookies.