Why does it say "Ki"?
Rashi: [Even though it is unreasonable to abandon such water], so My nation did - they forgot Me, and burn [offerings] to nothingness.
Why does it say "la'Shav Yekateru"?
Malbim: This is like "they carved out broken cisterns" (2:13).
Who or what caused them to stumble?
Radak: The verse omits this; it was the false Nevi'im. Also "va'Yaged l'Yakov" (Bereishis 48:2) omits who told him.
Malbim: They 1 put stumbling blocks on the paths, as if they are not easy to travel due to the stumbling blocks. In place of them, they entice them to go on unpaved paths.
Malbim does not specify who did so. Perhaps it is the gods to which they offer incense, or the false Nevi'im, even though they were not mentioned. (PF)
What are the "Derachim"?
Malbim: They are paths of Torah and Mitzvah; they are wide paths (Reshus ha'Rabim). They include the encompassing conduct in every time and place for the entire nation.
What are "Shevilei Olam"?
Rashi: They are fixed (proper) paths. They said that they are a stumbling block, in order to [justifywhy they] go on a path that is not paved.
Radak: They are the paths that our righteous ancestors went in. The verse omitted [that they caused us] 'to abandon' (the paths). Many verses omit words that are understood, e.g. "va'Techas Alecha" (Shmuel I, 24:10) is lacking 'Nafshi' or 'Eini', "va'Techal David" (Shmuel II, 13:39 is lacking 'Nefesh' - Radak there).
Malbim: These are paths of individuals, in all their individual deeds. They never change; this is why they are called Shevilei Olam.
What is "Derech Lo Selulah"?
Radak: It was not paved; it was a new path - "Chadashim mi'Karov Ba'u" (Devarim 32:17).
