One thing we know from our own competitive community and fighting games in general is how important mindset is. If you're nervous, have doubts, don't believe in yourself, feel outclassed, etc. these all play a part in you defeating yourself.
Having the right mental attitude goes a long way in fighting games, and in many other things in life.
We've seen upsets happen in both online and offline tournaments before. It doesn't happen all the time, of course, but it does happen.
For Kitana, while she served Shao for almost 10,000 years, she never fully believed in what she was doing. She always had doubts about how things were. She also feared Shao, "father" though he was.
When she stepped into the arena to challenge and defeat Shao, this is one of the first times we've seen her truly focused, truly committed, and truly believing in herself and the justness of what she was doing. She had the right mindset and made the upset happen.