Charles H. Spurgeon on Psalms 78:31

The wrath of God came upon them, and slew the fattest of them. Two things are here worthy of notice. 1. One, Why he gave them abundance and sufficiency of quails, and afterward punished the murmuring and unbelieving. If he had punished them before, he would have appeared to have had greater ability to destroy them, than to give them flesh. Therefore, that he might first declare his power, and so make the unbelief of the people the more plain, and show how deserving they were of punishment, he first showed he could give, because they believed he could not, and then punished them for their unbelief... 2. The other, that he destroyed the fat and the chosen men among the people, although they all are said to have murmured. Without a doubt, they were first in the crime, and therefore they are specially mentioned in the punishment. Musculus.