The Praetorian guard was essentially the secret service, but soon became king makers. The leadership of the guard was sick of being humiliated by emperor Caligula and did not like the way things were heading. So they assassinated Caligula and murdered his family.
The problem was that Caligula's family were the Julio Claudians, descendants of Julius Caesar (the now god), and the people of Rome weren't willing to accept a leader that wasn't of the royal family.
Luckily for them, they found goofy uncle Claudius hiding behind the curtains. Uncle Claudius was the son of Germanicus, one of Rome's greatest heroes, and uncle Claudius had kept himself alive by playing up his disability for the amusement of nephew Caligula (many believe Claudius had cerebral palsy). But now Claudius was the only male descendant of Caesar, so the guards hoisted him on their shoulders and declared him emperor against his will. The Praetorian thought he'd be easy to control, but instead he gutted them and killed the men who declared him emperor.
He became a pretty solid emperor, very much a scholar. And there's a fantastic series called I, Claudius about him.
Derek Jacobi as Claudius, John hurt as Caligula, and Patrick Stewart (with hair) as the Praetorian leader Sejanus. It's a masterpiece, but don't expect HBO Rome. I,Claudius is more like a play
Note that he avoided being emperor because of their short life spans and he was not an exception, as he is suspected to have been murdered by his wife.
The I, Claudius story (which takes some liberties) explicitly shows Livia (Mrs. Augustus, and the Cersei Baratheon of the story) disposing of anyone who might interfere with her own son inheriting the empire. Usually, she poisons them, and does it personally.
Claudius is presumed to be dull because he stutters, and he plays up the part to stay out of the interests and malice of Livia.
Eventually Calligula gets into power, which is a Joffrey and a half, and when they finally get around to assassinating him, the Praetoreans threaten coup d'etat unless Claudius is made Caesar (thinking he can be manipulated.) In fact, he's the Tyrion Lannister of the bunch.