Simple. IRGC is part of it or more like initiator of it. Revenues got siphoned off. 112 Billion USD in export revenue got diverted to IRGC led front companies and offshore accounts instead of sending that hard currency back to the Central Bank, They keep it outside the country to fund their "war chest" and run a parallel economy where revenues never reach central bank yet imports are paid from the very same bank to cause an artificial deficit. Mullahs never got the hint of what IRGC did right under their noses. IRGC, Larijani + Mokhber factions, current government all seem to have led this coordinated coup which they knew ISrael and US will try to capitalise so it will provide them the oppurtunity to grab the power under pretext of war against superpower. Is there otherwise anyother explanation to this that a country earning ~120 Billion USD just from oil export is suddenly facing inflation ? Remember IRGC led companies run 1000+ tanker fleet that Mullahs cant control. This is Russian style war economy model where the forces themselves can fund an elongated war without concerning with government. This is why I say that Iran is heading towards military run technocracy run on petro economy. IRGC started this probably somewhere under Soleimani times but they are using it now. Ideally for them, war will start so they can finish the coup.
Why are they doing it ? different people have different theories. Some Iranian-American scholars think IRGC is hardcore nationalist group dedicated to gaurd the nation, they are part of Iranian deep state of power factions, bureaucrats, intelligence community, business groups .... all of them want Mullahs gone because they might see Islamism as un-necessary or themselves as better qualified for running the nation. Ali Shabani of Amwaj thinks that IRGC's grievance started with Khamenei's resistance towards nukes and probably the fact that Soleimani led IRGC crafted resistance empire from Yemen to Lebanon is lost all because Khamenei could not nuke up the nation despite all the tools available to him at the right time. His loyal generals in IRGC, Artesh are gone now.