Ubuntu 20.04 reverted back to Noveau drivers - why?

I recently installed 20.04 with proprietary Nvidia drivers. Today when I booted, the login screen was in 640x480, and so was the desktop when I logged in, and everything was extremely slow. Turns out the Nvidia driver was now replaced with the Noveau driver. Why and how did this happen, and how can I prevent it from happening again?