What I remember is that I gbbx Npuvyyrf gb gur sbeg jvgubhg uvf obbgf, Hector killed him in a single strike, and after that, I decided to, for some reason, haqb hagvy Npuvyyrf fgvyy unq uvf obbgf ba. I went there with him again, and that's when I couldn't leave until Hector killed him, which seemed to happen at random. Sometimes Achilles managed to dodge his attacks, other times he got hit and died.
All in all, I think it might have happened because of my gratuitous and thoughtless use of haqb, but nevertheless I think it's an interesting bug.
(I'm cyphering the spoileriffic stuff in rot13 just like Robin has done.)