I think it has to do with how our brain interperts the data. Normally you judge the directions of a force by comparing it against gravity, which most of the time is the strongest constant acceleration on your body. When you get near or above that constant force, your brain has to guess which which force is “down”.

Consider this too, your’re being accelerated forward, a significant part of your spatial orientation is visual, everything is rushing towards you, so it’s probably an additional input that causes your brain to determine that you are falling forward instead of down or backwards.