Roughly speaking, if you push forward hard enough the plane goes down. There are limits to that of course, but in this case the FD wanted to maintain an altitude of 0 so it was "suggesting" a downwards pitch to the pilot, and the pilot was doing as told. The FD is only an advisory system, it basically lets the pilot choose a goal and then it shows on the attitude display where to point the nose to achieve that goal. It helps pilots to be more efficient and precise and, in theory, avoid errors. In this case though it was misleading and apparently the pilot did eventually realize that and stop following it, but very very late, after other indications like high airspeed and the view out the window would have strongly suggested that the climb was not going as intended.