Why does it say "Ki Nasan
Radak: This refers to the previous verse. The reason [for what occurred to us] is because [they know] that Hashem gave the land to us, and melted from fear.
Malbim: We know that Hashem gave the land to us, for we saw that He saved us miraculously. And we know that all the residents melted, from Melech Yericho's fear and pursuit of us, and from Rachav's words.