Completely innocent people also get banned by clientside systems. Accidentally left some programming tools open from work, particularly for debugging or reverse engineering? Expect a ban. Tried to run the game on an unsupported platform, e.g. via WINE? Ban: https://www.reddit.com/r/linux_gaming/comments/9uk38b/was_ba...
This is what happened to me on VAC. I was running some memory snooping tool for experimenting with an entirely unrelated process. It was simply minimized and I forgot it was running. At some point I then decided to play a game of TF2. A few days later the permaban hits with zero chance for recourse. I am far more open to a softer ban system where I can actually appeal a ban if something goes wrong.