Albert Barnes Commentary Psalms 78:31

"When the anger of God went up against them, And slew of the fattest of them, And smote down the young men of Israel." — Psalms 78:31 (ASV)

The wrath of God came upon them – See (Numbers 11:33).

And slew the fattest of them – literally, “slew among their fat ones.” That is, the most vigorous among them were cut down; the people most eminent for rank, for influence, for strength, for valor. How far this was the natural effect of indulgence in eating, and how far it was a direct miracle, cannot now be ascertained. In either case it would equally show the divine displeasure.

And smote down – Margin, as in Hebrew, “made to bow.” That is, they were made to bow in death.

The chosen men of Israel – Margin, “Young men.” The idea is that of select men; men who would be chosen from among the others; men distinguished for vigor or influence. Not the aged or the feeble particularly, not those who might be naturally expected to fall, but men of strength who might be supposed to be capable of resisting the ordinary attacks of disease. God showed in this way that the judgment came directly from his hand.