1 The people took to complaining bitterly before the LORD. The LORD heard and was incensed: a fire of the LORD broke out against them, ravaging the outskirts of the camp. 2 The people cried out to Moses. Moses prayed to the LORD, and the fire died down.
4 The riffraff in their midst felt a gluttonous craving; and then the Israelites wept and said, “If only we had meat to eat!
10 Moses heard the people weeping, every clan apart, each person at the entrance of his tent. The LORD was very angry, and Moses was distressed.
ויהי העם כמתאוננים, “The people were as if looking for a pretext to complain.” ...As the people entered the desert they experienced a variety of uncomfortable sensations. These complaints were very displeasing in the ears of the Lord, seeing they were caused by the fact that instead of marching joyfully towards their destiny and the Holy Land, the people marched only grudgingly. The reason the word מתאוננים is spelled with the prefix כ i.e. a preposition describing something in relative terms, i.e. “as if,” is because at that stage they did not yet dare verbalize their feelings of discontent. They did not want Moses to hear their complaint. This is why the Torah describes God as “hearing” unspoken complaints by the people. Moses had not heard these complaints.
Isaac Arama
"[In verse 1]...the subject of their grumblings is not revealed at this juncture, since, at the beginning, they kept it inside. Later, they clothed their discontent in the form of a hankering for meat [verse 4]; but though, at the beginning, their discontent smoldered within them, the Lord who knows the thoughts of man was well aware of what was going on inside their hearts.... The phrase 'The Lord heard' implies that He understood the situation, though nothing had actually been explicitly said...."
Isaac Arama
"The text notes that the people did not learn the lesson of the punishment, though they suffered decimation from the anger of the Lord (verse 1). They reverted to their grumblings...They did not lust for meat or any particular item of food but simply indulged in lusting (verse 4a). This is much worse than the case of a man who is overcome by his desires....When the people saw the hankering of the mixed multitude (riffraff) they began once more to weep and lust, as at the beginning. This time, however, they expressed their discontent openly...."
And the people were like mitonenim: ...When they distanced themselves from Mount Sinai, which was almost like a permanent encampment, and then, after their first journey, found themselves in the vast terrifying wilderness, they made themselves uncomfortable. They said, "What will we do, how will we live in this wilderness? What will we eat and drink? How can we handle this oppression? When will we leave [this wilderness]? ...Since the text says that the people were in pain and sorrow, it mentions and tells us their sin [explicitly], as they were speaking from the bitterness of their souls like people who are in pain do. And this was wicked in the eyes of God, for they should have followed Him with joy and good spirit from all of the good that God had bestowed upon them. Instead, they were like people being forced, like people in pain, like people who were annoyed at their poor situation. This is also why the Torah tells us a second time (in verse 4) that the Children of Israel returned and cried. Their first sin was that they were annoyed that they lacked enjoyable things in the wilderness. They then repeated this sin instead of receiving message of the fire of God that burned among them [after the first sin].