Sugarcoat gets very annoyed every time Applejack calls her "sugarcube". To her emotional instincts, it's considered as an insult about her name so she tries anything verbally to get Applejack to stop calling her that name. This will be really difficult for Applejack to deal with when it comes to being around her. Of course she doesn't mean to hurt anybody's feelings on purpose, especially Sugarcoat, who hates being called "sugarcube".