Autoplay. One of the single most stupid default settings in Windows ever since Windows supported removable media, which is a while ago. It's up there with the default of hiding full file names from you.
Basically, if you inserted the disc, Windows would see the data partition, and "helpfully" run the installer for you. Then you'd have the driver installed, and it would block access to read the disc as a generic music disc.
It was also a massive usability boost. The process of "insert installation CD" then "nothing happens" was a bad experience for an extreme segment of the user base.
In 1995 the idea of commercial products being adversarial is low. For the very few cases of the driver they install being unwanted, there's countlessly more cases of it being wanted.
This wasn't "secure by design," it was "usable by design." And for 99.9% of the people, it was likely both.
Basically, if you inserted the disc, Windows would see the data partition, and "helpfully" run the installer for you. Then you'd have the driver installed, and it would block access to read the disc as a generic music disc.