Since the DGMO( a very high ranking official within the PA) and DG ISPR( the mouthpiece of the PA) have said that it was intentional, so it definitely means that they have some reason to say so.
Now, neither I nor you nor anybody else on this forum is privy to those details, so my bet is , that the US administration has made no secret of their liking to go across the border in the pursuit of militants.
So, to check the response , they might have gone across the border, not originally planned to shoot down this many soldiers. They went about for some time, had a little hover and all. But then, they might have started to shoot around a little bit, they saw the PA post, shot around it for sometime, and they got the response in terms of small arms fire and other munition being spent. That is when things might have got a little heated. Then a fire fight might have broken out, and the casualties took place.
This is just one of the possible reasons NATO might have done it on purpose.