Yehuda Nachshoni’s “Studies in the Weekly Parsha” (pages 1032-1033) on our Parsha Korach cites Ramban’s view that the cause of the rebellions: Korach, Dasan and Aviram and the First Born’s was:
The spies’ severe punishment, which brought death to the generation of the desert and plague to its princes. It [the punishment]… brought to the surface all of the accumulated bitterness of the dissatisfied, who until now had not dared to come out against Moshe. Now they took advantage… to settle accounts.