I call this 'the pile on syndrome' whereby once one faction attacks and wins a battle, everyone else decides the pickings will be easy and of course they are when 6 nations decide to attack one. This happens in my games all the time. The most classic case is what happens to the HRE almost every game. The Mongol Horde often succumbs to the same tactic (of course it is their own fault for attacking everyone). I make it a high priority to make the most of these situations.