וַיִּלֹּ֧נוּ הָעָ֛ם עַל־מֹשֶׁ֥ה לֵּאמֹ֖ר מַה־נִּשְׁתֶּֽה׃
백성이 모세를 대하여 원망하여 가로되 우리가 무엇을 마실까 하매
Rashi on Exodus
וילנו AND THEY MURMURED — This is a Niphal form, and similarly the Targum is a Niphal form: ואתרעמו (the Ithpael form in Aramaic corresponds to the Hebrew Niphal). And such is the way in which the expression denoting murmuring (from root לון) is employed: to refer back the matter (the action of murmuring) to the person (the subject of the verb); thus one says מתלונן and מתרועם and one does not say לונן and רועם. So, too, the Frenchman says “decomplaisant se” in old French, where it turns back the matter (the action) to himself (the subject) by saying, “se”, English himself.
Or HaChaim on Exodus
וילונו העם על משה, The people murmured against Moses, etc. The Torah criticises the manner in which the people provoked a quarrel. The Torah does not deny the validity of a people asking for drinkable water, i.e. the question: "what are we to drink?" They should have asked for their needs in an appropriate manner.
Siftei Chakhamim
To revert the statement to the person: מתלונן . . . I.e., meaning: [It refers] to the complainer. But the expression לונן does not revert to the complainer as much.