For a while, I wondered why Shenzi or one of the others didn't threaten to expose Scar for murdering Mufasa when they were hungry and Scar callously told them to "Eat Zazu!".
I'm wondering, did they actually see that Mufasa was thrown by Scar to his death or did they think that Mufasa either got trampled directly or that he lost his grip on the rocks and fell to his death and that it was totally on them?
If so, it would explain why they didn't try the blackmail. Also, it would explain why they were SO angry when Scar called them the enemy and told Simba to blame them.
They realized, with his confession plus that remark, that they had been nothing but pawns and perhaps Scar had been even threatening to expose them for the murder they didn't commit (It would be just like him!) all of these years to keep them in line.
Now that it looked like he was going to try and tell the rightful king that they were at fault for Mufasa's death, now after knowing the truth, that they had had enough of him and planned to kill him (whether he won the fight with Simba or not, though I think they may have initially backed off because they thought Simba might do the job for them.)