ít's a demarcated border, it was probably an accident.....it was loitering within those 600 meters for about 10 minutes; probably somebody radio'd them and told them to vacate the zone
hot pursuit convention does apply, but in this case no hostile engagement was taking place
therefore it was either a mistake, or as some suggest, it was a 'test' of some sort; I personally don't think that it was intentional, simply because Pakistan already showed that it is willing to close the border in the event of 'unfriendly' behaviour and failure to abide by rules -- it wont make the NATO look very efficient or accurate if they are not keeping track of their coordinates and location.
something needs to be done to better marcate the border, though this is pretty much close to IMPOSSIBLE task