How does Hitagi fall if she's weightless? At the start of Hitagi Crab


  in an accident involving a misplaced banana peel, Hitagi falls down the flight of stairs that lead to the upper floors of Naoetsu Private High School and ends up in the arms of Koyomi Araragi




but the oddity that Hitagi was inflicted by was the Heavy Stone Crab which took away her weight which means she's weightless and sorta like being in space so when she tripped she wouldn't fall down like unless someone gave her a push after. 

not to mention that when Araragi catches her he feels that she's weightless which is how Hitagi's "secret" was found out by him

How does Hitagi fall if she's weightless?
As @AkiTanaka mentioned and by checking Volume 1 of Bakemonogatari, we can see that she's not exactly weightless. She weighed 5 kilograms. To quote from the light novel:


  You’ve noticed, haven’t you?...That’s right─I don’t weigh a thing..Well, it’s not as if I don’t at all─a girl my height and body type would have an average body weight in the high forties, in kilograms...But my actual body weight is five kilograms...Now, if you want to be completely accurate, it’s only that a scale displays my weight as five kilos─I personally don’t notice it....


Her saying she weighs nothing is just an exaggeration or a figure of speech. She doesn't really weigh nothing, it's more like a hyperbole, since the difference between 5kg and an average person's weight is noticeable.