I think the fundamental cause is that players aren't just players, they're also people. And people have all sorts of problems. The distance in elite sport between being excellent and being mediocre is very small. If anything occurs to knock your concentration, or your confidence, or your fitness, or any sort of physical or mental well-being, then it can make all the difference. And as for how you get that level back again, well, it depends on the situation and on the person... some will get back to their best, some won't. For Alli this move to Everton is a great opportunity, he can help to save them from relegation and become a hero... will it happen? We have no idea. Maybe his first game for them will give us some clues where he is physically and mentally.