Google SERPs are observed at some instances like a keyword is typed into the search box, to be in a tree form of links directing to different pages of the site such as careers, news etc.
But the site owner may not have control over links right under the first result for other things on the site. There is nothing that one can deliberately do to get the search engine to do this with a specific site.
Some say that Google has decided that a site generates lots of search activity and it is their way of providing the most relevant results and Google determines that more than one page on a site is a good "match" to the keyword phrase searched for, it may give the multiple listings. Also, some others say that it is purely G's white list: Fortune 1000+ corps of American sites Top ~200 Universities sites.
Finally, You could call it as algorithms; I guess just a little simple one. For example for a keyword “BW Systems” a site that is poorly optimized is listed in a tree form which is none other than the popular BW Systems given priority by Google.
