This is a power grab - a coup d'état by PDA and Mullin, who had been biding their time whispering into Vivek's ear about Malone's strategy, rotations and player development, waiting for just the right moment to pounce. No way does Vivek make this move on his own. Either PDA and Mullin were actively advocating for Malone's firing, or they were complicit by not having Malone's back - a non-verbal shrug here, a grimace there, or a dismayed shake of the head in response to a Vivek inquiry about Malone's coaching. The mysteries of why Sessions would play over Collison or Stauskas over McLemore in key minutes of a 4th quarter make sense now. A coach doesn't do that unless he's instructed to do so.
I find it hard to believe that George Karl would take over for Malone, unless he could be afforded considerably more power within the organization than just head coach. Karl isn't a stupid man. When he sees the head lopped off of one of his brethren for no good reason, he's going to want to make sure that his is firmly protected by an iron collar. PDA and Mullin certainly aren't going to want that. If Karl is hired with substantial power in the organization it would be because Vivek did a double-cross on PDA and Mullin and at time of Vivek's choosing he will have them both guillotined.