Hello Kinson, I have had a look at the lyrics and I am not 100% sure but if you describe a person as a wallflower it means a very introverted person. A back bedroom is a (usually) smaller room where less favoured guests or children sleep. So a back bedroom casualty would possibly be a person who is introverted because they lived in the back bedroom and not part of mainstream family life. This is my take on it but if someone comes up with a better answer they will probably be right.