In order to avoid programmed favourites. I mean, a script (a small program) that is capable to favourite a determined amount of favs automatically. Because the system can't indetify if the order comes from a human or from a program, they set a limit of faves per time that if you surpass you can't fav in an amount of time. They raise this limit some time ago. Before, I have also that issue, but since they raised, I have never have this problem.
sorry for my english.