I would suggest grabbing the demorec from http://139.99.139.128/ if you know the date/time/round. This will allow you to replay the action from both sides. More than likely you will find there is a slight desync between the views which is not uncommon. I think you will find the shot was made before he crossed the tree from his view but from Necrotiks it looked different. It could also be possible that - that particular tree isn't picking up on collisions. This is something we can test in game, or if I remember I'll look in the beta version too see what it shows me.
Not saying it was cheating. It's reasonable for a good player to look at the upper bastion before the last mad dash, and if necessary, take a shot. I reckon there was a lot of luck in the hit, but it's not impossible.
