While I have not seen this movie in a while, I will try my best to explain as this scene is fuzzy in my memory.
If I recall correctly, this is right before they encounter the rest of the Order in the Department of Mysteries.
One of a few things could have happened here:
- Due to the effects of the strange objects in this Department, it is possible that they stopped the fall of the group, allowing them to land safely.
- In the book, speechless spells are a thing, and are not mentioned as well in the movie, but still happen. It is also possible one of the group (possibly Hermione) used a speechless incantation of Arresto Momentum to stop the groups fall.
- The third reason is that since the Order members are awaiting Harry's group, they could've noticed the fall from a distance (assuming they didn't just Apparate into the Department) and used the spell off screen.
- It is a simple plot hole not thought out by the directors.