Explanation: In the aptly named Year of the Five Emperors, the Emperor Pertinax, a GOOD AND HONEST AND NOBLE EMPEROR, was brutally murdered by the Praetorian Guard, the supposed bodyguard of the Emperor, for [checks notes] not bribing them enough. The Praetorian Guard then sold the position of Emperor to the highest bidder - one Didius Julianus, who paid 25,000 sesterces (roughly 20 years' pay for a common soldier) per Praetorian in exchange for the Imperial position. And there were around 8000 Praetorians.
Didius Julianus was overthrown a few months later by Septimius Severus (who also defeated his other two rivals for the Imperial position). Predictably, the Praetorian Guard sold Didius Julianus out at the first opportunity. Severus, wary of the backstabbing fucks, had the Praetorians surrounded by his troops, stripped of their positions, and then replaced with veterans from his loyal legions.