I think that if a tree suddenly appeared where the collision happened, the offense (green) could have avoided it, but the defense (black) wouldn’t have been able to avoid it because he was running forward without looking forward… Or if you want, you can see the player in green as the tree that appears right there to where black was going, obviously, he could not avoid it.
Defense is running along with the disk to catch it while it comes closer to the ground, but ofense makes a turn and runs towards the disc and the reserved space of the other player, or at least that is what I see.