The use of the word "violent" is very interesting when describing Mr. Bingley's love. This word is defined as, "By or with great force, strength, or vigour; with a violent motion or action; so as to produce a violent or powerful effect" (OED). This definition shows that his love for her is really intense, showing that he's deeply in love with her. Instead of saying that he is deeply in love with Jane, the use of "violent" works in this context because he does end up hurting her emotionally. This is a play on words used by Austen on purpose. With this being said, Mrs. Gardiner states that the phrase is "hackneyed" which defines as "To overuse or make too familiar; (hence) to make trite, banal, or uninteresting, esp. through indiscriminate use" (OED), which again plays with how Mr. Bingley hurt Jane. Both definitions show that Mrs. Gardiner is not fully convinced of Mr. Bingley loving Jane, that it seems fake.