“They’re the cause”.
This story is about the fragrance of flowers that can make one want to leave the room, and easily put them in a bad mood. Aroma can cause discomfort too, esp. strong fragrances. Just like how a few people are chemical intolerant and a few others are allergic to pollens, there is a set of people for whom the fragrance of flowers can trigger severe headaches to migraines.
Yeah, flowers are pretty but painful. Their softness can hurt too!