Garrosh... has had a slow descent into madness since wrath. It Started with his anger at Varian being present at the meeting in the violet tower ( Ulduar patch cutscene/trailer). His rage builds slowly, over time.
This is due to his pride. Remember, when we FIRST met Garrosh, he was a whimpering baby, unable to cope with ANYTHING becuase his grandmother was dying in Nagrand. you do quest after quest after quest trying to cheer him up. Eventually Thrall Himself comes to outland, and meets with Garrosh, and it's his tales of His father, told by Thrall that finaly spurs him to action, Using his Pride.
Pride. Since Day one, Garrosh has been about Pride. Pride in his Father. Pride in his Orcish heritage. And when something wounds his pride, he gets angry.
To paraphrase SaoHao " When Pride is good, it is VERY good... but when it is Bad, it is the Worst, Most insidious thing"
Cataclysim: Garrosh ends up fighting a duel to the death, Which is the Old orcish way ( Pride in his heritage. Pride in his ability) with Cairne Bloodhoof for leadership of the horde. This victory is stolen fro him by the misplaced trust he put in Magda Grimtotem, who used the opportunity to Murder Cairne by posining Garrosh's weapon.
Having learned of the circumstances that led to him killing Cairne, Garrosh sends this reply:
Unto Elder Crone Magatha of the Grimtotem,
Acting Warchief of the Horde, Garrosh Hellscream,
Sends his most sincere wishes for a slow and painful death.
It has come to my attention that you have deprived me of a rightful kill. Cairne Bloodhoof was a hero to the Horde and an honorable member of a usually honorable race. It is with disgust and anger that I discover you have caused me to bring about his death through accidental treachery.
Such tactics may work well for your renegade, honorless tribe and Alliance scum, but I despise them. It was my wish to fight Cairne fairly, and win or lose by my own skill or lack of it. Now I shall never know, and the cry of traitor will dog my steps until such time as I can sport your head on a pike and point to you as the real traitor.
So...no. I will not be sending any truehearted orcs to fight alongside your treacherous, belly-crawling tribe. Your victory or your defeat is in the hands of your Earth Mother now. Either way, I look forward to hearing of your demise.
You are on your own, Magatha, as friendless and disliked as you have ever been. Perhaps more. Enjoy your loneliness.
This FURTHER Injured His pride, pushing him closer to madness...
The events of Cataclysim, Leading into Mysts Pushed Garrosh's pride SO FAR, that He called the Sha of Pride itself to him, which had two results.
1) It Corrupted Garrosh irredeamably, leading to his insanity during the Mysts Expansion.
2) the Sha of pride had been turned into the Mysts that sorruneded Pandaria By the last Emperor. When It Found a more suitable host in Garrosh, this is what caused the Mysts to part.
Once he was in the grip of the Sha of pride, it was Over.
It's not a "sudden thing" Garrosh has been building to a breaking point for a while.