The Ramban remarked that it is far-fetched to assume insurrections took place at Sinai while the nation's situation was positive and Moshe's leadership was unsullied and expected to imminently result in the nation entering the Promised Land.
Grievances often remain suppressed until the opportunity presents itself for release.
And the order of the Torah should not be assumed to be non-chronological except where clearly indicated, and there is no such indication in our text. The Ramban also disagreed with Ibn Ezra regarding the basis of some of the grievances. One agenda sought the overturning of the religious establishment and focused on Aharon's position and the other was directed against Moshe's leadership. As the Ramban states, this is a usage to describe one who bestirs himself to begin something. Qorah was here the initiator.