Mahomes is moving to his right on the run. Balls going to move right too.
Check out this one by Rodgers to Cook. Rodgers running left, ball "curves" left. Go to about the :33 to :36 second mark shows the curve. Earlier to see Rodgers running left.
http://nfl.com/videos/nfl-cant-miss-...e-pass-to-Cook
Think of it this way. If you are driving down the road at 60 mph, and throw something out of your vehicle perpendicular to the direction you are going, that object is still going 60 mph in the direction you are driving, along with the perpendicular direction. It's going to have a curve to it until it hits the ground.