How does a computer security vulnerability occur?

Vulnerabilities exist because of exploits in code or networking protocols. Millions of lines of code are required to make an operating system, and sometimes vulnerabilities can be found within. It's no different than having a house and accidentally leaving a window open for a burglar to get in easily.

Unfortunately, many computer science curricula concentrate on coding projects without regard for coding securely. Security as an afterthought is not as effective as incorporating security in the beginning.