Because Hermione was too clever

You might have assumed that knowing an awful lot of about the wizarding world is a good thing. Unfortunately not when you’re Hermione, and certainly not when your impeccable knowledge contradicts Professor Snape’s insults…

‘Well, well, well, I never thought I’d meet a third-year class who wouldn’t even recognise a werewolf when they saw one. I shall make a point of informing Professor Dumbledore how very behind you all are ...’

‘Please, sir,’ said Hermione, whose hand was still in the air, ‘the werewolf differs from the true wolf in several small ways. The snout of the werewolf –’
‘That is the second time you have spoken out of turn, Miss Granger,’ said Snape coolly. ‘Five more points from Gryffindor for being an insufferable know-it-all.’

Harry Potter and the Prisoner of Azkaban

Because Harry wasn’t being punished enough…

After a bountiful number of detention nights with Professor Umbridge, it was fair to say the punishments hadn’t exactly transformed Harry into a calmer member of wizarding society. Thankfully, Professor McGonagall had a trick for this: rubbing salt into the wound.

‘Potter, you must get a grip on yourself! You are heading for serious trouble! Another five points from Gryffindor!’
‘But – what –? Professor, no!’ Harry said, furious at this injustice.
‘I’m already being punished by her, why do you have to take points as well?’
‘Because detentions do not appear to have any effect on you whatsoever!’ said Professor McGonagall tartly.

Harry Potter and the Order of the Phoenix

Because Malfoy just had too much heart

If you had a Sickle for every time Professor Snape pettily took house points away from Harry, Ron and Hermione, you’d probably have enough money for a couple of Firebolts and a spot of lunch at Madam Puddifoot’s teashop. However, on occasion, maybe the Professor had good reason.

Malfoy spent much of their next Potions class doing Dementor imitations across the dungeon; Ron finally cracked, flinging a large, slippery crocodile heart at Malfoy, which hit him in the face and caused Snape to take fifty points from Gryffindor.

Harry Potter and the Prisoner of Azkaban

Because of Ron’s unorthodox method of spotting ghosts

When Professor Snape finally took over as Defence Against the Dark Arts teacher, he quickly found something else he needs to defend himself against: Ron’s comebacks.

‘The Inferius is a corpse that has been reanimated by a Dark wizard’s spells. It is not alive, it is merely used like a puppet to do the wizard’s bidding. A ghost, as I trust that you are all aware by now, is the imprint of a departed soul left upon the earth … and of course, as Potter so wisely tells us, transparent.’
‘Well, what Harry said is the most useful if we’re trying to tell them apart!’ said Ron. ‘When we come face to face with one down a dark alley we’re going to be having a shufti to see if it’s solid, aren’t we, we’re not going to be asking, “Excuse me, are you the imprint of a departed soul?”’
There was a ripple of laughter, instantly quelled by the look Snape gave the class.
‘Another ten points from Gryffindor,’ said Snape.

Harry Potter and the Half-Blood Prince

Because Malfoy was in charge

When Professor Umbridge gave Draco Malfoy the right to deduct house points from students, the result were akin to inviting a dragon to your newly-built log cabin.

‘The Inquisitorial Squad, Granger,’ said Malfoy, pointing towards a tiny silver ‘I’ on his robes just beneath his prefect’s badge.

‘A select group of students who are supportive of the Ministry of Magic, hand-picked by Professor Umbridge. Anyway, members of the Inquisitorial Squad do have the power to dock points … so, Granger, I’ll have five from you for being rude about our new Headmistress. Macmillan, five for contradicting me. Five because I don’t like you, Potter. Weasley, your shirt’s untucked, so I’ll have another five for that. Oh yeah, I forgot, you’re a Mudblood, Granger, so ten off for that.’

Harry Potter and the Order of the Phoenix