It can also be considered rude to use the more formal style of speech when the social hierarchy dictates the informal style should be used.
The book Using Korean[1] gives a detailed explanation of how formal speech indicates social distance more than simple politeness:
> [존댓말] indicates a psychological distance between the speaker and the hearer... a couple in a romantic relationship who normally use an intimate casual style with each other will suddenly switch to a formal style after they fight, to demonstrate the distance they feel from each other.
The book Using Korean[1] gives a detailed explanation of how formal speech indicates social distance more than simple politeness:
> [존댓말] indicates a psychological distance between the speaker and the hearer... a couple in a romantic relationship who normally use an intimate casual style with each other will suddenly switch to a formal style after they fight, to demonstrate the distance they feel from each other.
From a related discussion: https://www.reddit.com/r/Korean/comments/vcusut/comment/icj2...
[1]: https://books.google.co.kr/books?id=2ggVsnUCbiAC&lpg=PA17&pg...