It means to do something intentionally that people don't expect. It isn't an accident--the person subverting expectations is usually trying to do something out of the ordinary. So, if everyone thinks I will become a doctor, but I decide to become a painter instead, I am subverting their expectations. It's more commonly used in art and entertainment. An action movie about cars and robots might subvert expectations and have a female main character. It's doing something different from usual, since most action movies are about men. A painter could subvert expectations by using unusual colors or techniques. It isn't a very common term in everyday conversation, but it is really useful when talking about art and literature. I hope this will help!

Here's the definition from the Urban Dictionary: subvert expectations To behave contrary to an established belief or assumption for the purpose of being fresh and interesting. Usually used in the arts when analyzing the reaction of the audience to a performance or piece of writing. Sometimes mistakenly used as a positive when the result in neither fresh nor interesting. A. Star Wars: The Last Jedi did a great job of subverting audience expectations! B. No, it was just a narrative mess. A. Your report written in crayon was very unprofessional. B. Come on! I was just trying to subvert expectations! Here's the definition from vocabulary.com When you subvert something, your words or actions criticize or undermine the usual way of doing something or common values. The girl who wears a tuxedo to the prom might subvert traditional ideas about beauty. To subvert an institution like a school or a government is to overthrow it or stop its normal way of functioning. So you can imagine something that subverts as overturning or flipping the usual way of doing things, like a student who subverts a teacher's authority, causing chaos in the classroom. This is not a term or phrase that is commonly used. I have never used it, although I have seen it in books a few times.