I can't link to the comment as it's in a close group on Facebook (Pool is Alive).
Someone captured the moment. Still hard to tell what exactly happened.
My guess: the chalk was right behind the 9, in Niels' eyesight. The chalk got in his mind and he quickly wanted to get rid of it so he just threw it away.
There might be some annoyance for leaving the chalk there, but he did not throw the chalk at him, or even close.