I just reinstalled Tor and the default search engine is not Startpage, it is DISCONNECT Search. Did I install a bogus Tor browser? And if not, why the change from Startpage?
You did most likely not download a bogus tor browser bundle if you downloaded from official download links, some time back they switched.
Our default search provider has also been changed to Disconnect. Disconnect provides private Google search results to Tor users without Captchas or bans.
If you still have concerns then download again and verify your download.
You most probably didn't download a bogus version of Tor Browser Bundle, but some 4.5 (or later) version. As Stig Atle wrote, the developers changed the default search engine.
You can easily move back to StartPage. At the search window (Ctrl+k) there is a small arrow right of the Disconnect D icon. If you click on it a menu opens and shows you all possibilities. Just choose StartPage and every search in this sessions uses StartPage.