In contrast to earlier occasions when the people complained about specific problems-- such as lack of food or water, or the need for a "god" to take Moshes place as an intermediary between God and the Jewish people, in the sidrah, there is an outright rebellion, an attempt to overthrow Moshe and Aharon as the leaders of the nation. The leader of the rebellion was their cousin and fellow Levite, Korach. As is typical of would-be tyrants who must attract a popular following to succeed, Korach posed as a champion of the masses and tried to discredit Moshe. Ibn Ezra and Ramban agree that the rebellion happened about a year after Egypt, but they disagree regarding exactly when it took place and the underlying reason for the rebellion. Their views are as follows:
Ibn Ezra- Korach rebelled right after the inauguration of the Mishkan, when Aharon and his sons were designated to replace the firstborn, as the only ones who would perform the sacrificial service. This angered Korach, who was himself a firstborn, and it was easy for him to enlist two hundred and fifty leaders of the assembly, who were also firstborn. Datan, Aviram, and On (pronounced own) were from the tribe of Reuven, which had it's own reason for resentment, having lost it's privileged firstborn status to the offspring of Yosef. Ibn Ezra assumes further that other Levites were upset at having been relegated to be mere assistants of the Kohanim. According to this view that dates the revolt soon after the Mishkan was erected on the first of Nissan, in the second year after Egypt, it happened after the events recorded in chapter 8, and there is no apparent reason why the Torah does not record it in chronological order.
Ramban- Although the Chachamim teach that the Torah does not always follow chronological order, ordinarily one must assume that the events took place in the order in which the Torah records them-- unless there is a compelling reason to assume that a particular passage is out of order. In Korachs case therefore, we must say his rebellion took place after the incident of the spies because people were resentful of the decree that everyone over twenty would die in the wilderness. Korachs own resentment began earlier when Aharon was made Kohan Gadol, or when their cousin Elizaphan was placed in charge of the Kohanite family, thus making him Korachs superior, and giving him a position that Korach felt should have been his. At that time, however, Korach did not dare criticize Moshe, who was so beloved by the people that they would not have tolerated a rival to him. But after the fiasco of the spies and the decree that an entire generation would die in the wilderness without ever seeing the land, Korach took advantage of the national dissatisfaction to foment rebellion.