On flop warning, Shelvin Mack said Brandon Knight stepped on foot causing fall. Was "shocked" by warning. #ATLHawks— Chris Vivlamore (@ajchawks) January 27, 2014
If you watch the video of the incident closely, you can tell that Brandon Knight stepped on Mack's foot, legitimately sending him to the floor.
Since it was the first warning for Mack, he wasn't fined, so overturning it just means that the hypothetical next flop he gets would also be fine-less. Justice for even the Hawks!