John Gill Commentary Exodus 16:20

"Notwithstanding they hearkened not unto Moses; but some of them left of it until the morning, and it bred worms, and became foul: and Moses was wroth with them." — Exodus 16:20 (ASV)

Notwithstanding they did not listen to Moses
That is, not all of them, some of them did, and perhaps the far greater part of them:

but some of them left it until the morning ;
out of distrust, for fear they should have none the next day; being men of little faith, that could not trust God for a supply for the morrow; the Targum of Jonathan says, these were Dathan and Abiram; and so Jarchi:

and it bred worms, and stank ;
or by an "hysteron proteron", and transposition of the words, the sense may be, that it stank, corrupted, and putrefied, and so produced worms, in which order the words lie, (Exodus 16:24) .

And this was not from the nature of the manna to breed worms so soon, but God so ordering and disposing it, that it should do so; for otherwise it would keep to another day, as what was gathered on the sixth day kept to the seventh, and there was a part of it kept for many ages, see (Exodus 16:24–33) , and since the manna was of the meal kind, perhaps those worms it bred might be of the weevil sort, as Scheuchzer conjectures ^F19 :

and Moses was angry with them ;
for breaking the commandment of God.

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FOOTNOTES:

• F19: Physica Sacra, vol. 2. p. 179.