What is the historical origin of the section titles of the Haggadah?
Machzor Vitry (sec. 65) attributes them to Rashi. This site says that it has also been attributed to R. Shmuel of Falaise, one of the Tosafists (mid-13th century), but that the true authorship is unknown.
(It was actually originally just one of a lot of mnemonics for the order of the Haggadah composed by various rishonim. Another one, from Maharam Rothenburg, is printed in Hagahos Maimonios in the standard editions of Rambam, end of Hil. Chametz Umatzah. R. M.M. Kasher's Haggadah Shleimah has these two plus a dozen or so others. Apparently this one won out because of its simplicity and brevity.)