She was always at odds with Snape. The war had ended, Voldemort was dead, Snape had survived Nagini's attack and was back to teaching Potions, and Harrie was butting heads with him regularly. He always found the pettiest reasons to criticize her work. Potter, your stirring rod is at an angle, five points from Gryffindor. Potter, you added your asphodel powder incorrectly. Five points from Gryffindor.