They were spun out of Sony BMG or whatever the music publishing arm of Sony is called. The rootkit thing happened in the early 2000's when the music industry were in full panic and tried to find ways to force people to buy and listen to CD's even though iPods, Zune and other were very popular and becoming bigger and bigger.
Selling music digitally would have been a good solution but they were stuck in the past, just like when the music industry tried to get rid of the 78rpm records because it ate profits of live performances, then tried to get rid of radio because it ate into the profits from selling 78rpm records, then tried to ban vinyl because it ate the profits from radioplay and so on.
They really drop the ball a lot, they are collectively the dumbest most backwards industry that have ever existed, they could have followed the developments of the market and the consumer, but nah, fuck that and fuck learning from all the times in the past that they have fucked up in exactly the same way.
So Sony developed an auto-installing rootkit for PC's to prevent people from ripping the music to their iPod's etc, something that only affects people that BUY cd's. Being a rootkit it couldn't be uninstalled either because its presence was secret, so that's nice. They put it on every cd they sold and while it didn't affect regular cd-players it didn't affect pirates that ripped and uploaded music either(I don't think it even worked on Mac and absolutely not Linux). Their actual consumers were the ones that got fucked. Sony also got found out immediately and the rootkit itself was a security hazard that easily allowed trojans and viruses to piggyback on it. Good times for Sony, what a shit show that was, not even StarForce got that much bad press. They released an uninstaller later, they had to, people were not happy, but iirc there was something weird with that one as well, not by actual malice but by incompetence that comes from hastily cobbling together an uninstaller for something that was never meant to be uninstalled.
Sony later turned their DRM division out into its own entity and that's Denuvo. This is all from memory and it's pretty old, but that's how I remember it.