Although the N1 (fan section) of the engine may run in the opposite direction to normal in a strong tailwind with the engine shut down I do not believe that the N2 or N3 section , as appropriate, also run backwards since these drive ancillary gearboxes and therefore have significant mechanical drag to overcome. It is the slowing of the N1 and then rotation of the N1 in the correct sense that gives the impression of the starter being worked harder. It is simply running for longer!