Something that's incongruous is inconsistent or incompatible with something else. Remember that Sesame Street song "One of these things is not like the other"? They were talking about that one thing being incongruous.
If we break it down into its Latin roots, incongruous is formed by adding in, meaning “not," to congruous, which means “suitable, proper.” So, something that is incongruous is "not suitable or proper" — in other words, not in harmony with everything else. That's why burping at the dinner table could be called incongruous behavior: it's not proper and certainly doesn't go well with the main course.