Albert Barnes Commentary Psalms 106:28

"They joined themselves also unto Baal-peor, And ate the sacrifices of the dead." — Psalms 106:28 (ASV)

They joined themselves also to Baal-peor—They joined in their devotions, or, they shared in the rites of idolatrous worship. This occurred when they were in the regions of Moab, and on the very borders of the promised land (Numbers 25). Many other instances of a similar kind are passed over by the psalmist, and this seems to have been selected because of its special aggravation, and to show the general character of the nation.

Even after their long-continued enjoyment of the favor and protection of God—after he had conducted them safely through the wilderness, after he had brought them to the very border of the land of Canaan, and all his promises were about to be fulfilled—they still showed a disposition to depart from God. Baal-peor was an idol of the Moabites, in whose worship females prostituted themselves. Gesenius, Lexicon. . Baal was the name of the idol; Peor was the name of a mountain in Moab, where the idol was worshipped.

And ate the sacrifices of the dead—Of false gods, represented as “dead” or having no life, in contrast to the true and “living God.” They ate the sacrifices offered to those idols; that is, they participated in their worship (Numbers 25:2).