What is "Chalaklakos"?
Rashi: It is a slippery land on which the foot slips and the person falls.
Why does it say "ka'Chalaklakos ba'Afelah"?
Rashi: This is stumbling after stumbling. Slippery land causes stumbling even when it is light, and darkness causes stumbling even when it is not slippery.
Radak: When it is dark, he does not see where to support himself, and where to walk.
Malbim: One who walks on a slippery land during the day can be careful not to fall, but if he goes in darkness, he will fall. If the Navi and Kohen would show to them the path, they would be careful. However, they did not illuminate for them, and they remained led astray in the dark.
Why does it say "Yidachu v'Naflu Vah"?
Radak: It is as if someone pushes them from in back of them. In any case they will fall. The verses mention three reasons for falling, to teach how quickly it will be - slippery land, darkness and being pushed. 1
Radak: The accent on Yidachu is on the penultimate syllable, unlike the norm for verbs in which the last letter of the root is silent.
Why is this the time to bring their punishment?
Malbim: Their measure for sin was filled.
