Teams in the middle now have enough class to damage "better" teams on the break. Stoke are something like the second lowest scorers in the division but press up extremely high on them and they've got players to do big damage. But then go on the next week to lose to Palace or lucky to get 1 point against us.
Think confidence plays a big part too, with the big teams losing that aura of being untouchable, Man U in particular. I imagine Leicester being top inspires players/managers as well.