Whose honor did they exchange?
Radak: Hashem's; they saw His honor among them on Har Sinai.
Malbim: They switched their honor, i.e. Moshe's conduct, for the form of an ox. Idolatry is based on the belief that Hashem handed over conduct to the Ma'arachah and powers of nature. Therefore, they tried to be pleasing to those powers, like they learned from the Egyptians.
Why does it say "Ochel Esev"?
Rashi (from Shemos Rabah 4:4): Nothing is more disgusting than an ox that eats grass. It excretes much and gets filthy from it.
Rashi (Shemos 32:5): It had life 1 , and ate grass.
Radak: An ox eats grass; the image that they made in its form did not.
Shemos Rabah (1:3): Witches of the "Erev Rav" made it via witchcraft.
