Why do managers struggle after stepping up to 'big-six' clubs?

Former Bournemouth boss Andoni Iraola (left) is set to join Liverpool, while Thomas Frank moved to Tottenham from Brentford [Getty Images]Moving from a successful Premier League side to a so-called 'big-six' club is sometimes seen as the natural next step for a manager. By the 'big six' we are referring to Arsenal, Chelsea, Liverpool, Manchester City, Manchester United and Tottenham. We do not use that term to disparage the likes of Aston Villa or Newcastle who have qualified for the Champions L