If you stand in the wind it will be really quick to knock you over. So quick that you probably won't even know the wind did it. You'll end up standing up again just to be knocked over again, just as quick.
The wind was so quick to knock you down that you sat (さつ) down instead. The wind may be quick, but it can't knock you down if you've already sat yourself.
Currently there are no similar looking kanji which exist in the WaniKani learning system