What is the significance of seeing high hills and thick trees?
Malbim: They saw what the Kena'anim do, to burn on Bamos to idolatry. They made the Bamos on high hills and under thick trees, like the rite of idolaters.
What is "Ka'as Korbanan"?
Radak: They angered Me with their Korban that they offered to Hashem, like the protocol of the Goyim that I dispossessed in front of them.
Malbim: They served jointly Hashem and idolatry. "Va'Yizbechu Sham Es Zivcheihem" to Hashem, and "Ka'as Korbanan" was to idolatry, to anger Hashem. "Va'Yasimu Sham Re'ach Nichocheihem" to Hashem, and "va'Yasichu Sham Es Niskeihem" was to idolatry, Their way was to pour libations - "Yishtu Yayin Nesicham" (Devarim 32:38).
