There was more than one incident; and the reason for each is
given when it happened. Some people complained (Numbers 11:1), some
complained about the lack of meat (ibid. 11:4), Miriam said
something not sufficiently respectful concerning Moses (ibid.
ch.12), the spies complained about the seeming impossibility of the
Israelites entering the Holy Land (ibid. ch.13), one person
desecrated the Sabbath (ibid. ch.15), Korach complained about the
leadership of Moses and Aaron (ibid, ch.16), some people complained
after Korach was punished by God (ibid. ch.17), some people
complained when there was no water (ibid. ch.20), some people
complained about the manna and the long journey (ibid. ch.21), and
some people went astray after the Moabite young ladies
(ibid.ch.25).