What happened to Liam when he was five-years-old in the novel, King of the Screwups?

In Chapter Two, Liam remembers back to when he was five years old. He is backstage at a runway show with his mom and the other models. He starts to imitate the models and they all start talking about how GQ he is. Liam ramps up his showing off and when he jumps off the stage expecting his mom to catch him, she doesn't and he falls into a pile of chairs.