I'm testing this now and I have not been able to recreate it as he does. If I aim low, then throw the cursor up quickly and fire, at time a single turret will shoot at the previous point, but the others will not.
I've escalated it to see what's happening.
EDIT: Ok, I was able to recreate it, but I had to aim VERY short and then throw my cursor up much further and fire almost at the exact same time. I suspect this is due to the fact that barrels due have to elevate in order to shoot - while a mouse cursor can have it's sensitivity turned way up to be almost instantaneous