The incident that started it all.
Snape's Third Year - Just before the Sorting Feast
Severus glared at the four boys in front of him, the stupid Gryffindors had played another prank on him, and they hadn't even been back at school for six minutes.
Brushing his goop covered hair out of his eyes he raised his wand, ready to attack.
But Black was faster, "Pyroclentia!"
The Slytherin's eyes widened in pain as his stomach began to burn, he heard Potter's hesitant and frightened voice as he hunched over, clutching his stomach.
"Sirius! The red skin curse is Plycoclentia, not Pyroclentia, what did you just do?"
"I-I dunno…"
Severus suddenly felt the overwhelming urge to belch. But when he did it was not a normal belch, fire shot fifty feet from his mouth, right towards the Sorting Hat McGonagall was carrying into the Great Hall.
A shocked silence fell over the crowd as they watched the hat begin to burn.
"AAAAAAAH!" Hopping down from his place in McGonagall's arms the hat began making laps around the hall, apparently trying to blow the flames out with the force of it's hops, but it only succeeded in catching the students' robes on fire.
The students all began to panic as the flames licked up their robes, and many began to pull them off, revealing more burning clothing underneath.
Meanwhile Severus and the Marauders just watched the whole spectacle from the center of the room with something like awe in their eyes.
Then they began to snicker. This proved to be a very foolish thing to do for suddenly the hat stopped screaming and bouncing and starred at them with hatred.
"YOU!" It shouted before it lunged at the slime covered Slytherin, catching his hair on fire as it bit him with the rip in its brim.
"GET IT OFF ME! GET IT OFF ME!"
"YOU SHALL PAY FOR YOUR ATTEMPT TO MURDER ME SNAPE!"
"GET IT OFF! GET IT OFF! SOMEONE PLEASE GET IT OFF!"
Finally Dumbledor snapped out of his shocked daze and muttered a spell that lifted the hat off poor boy as McGonagall drenched the lad in an attempt to put out the flames that licked at his hair.
When they were finished the hall stared at him, half his hair was missing, and the part he had was still covered in green gunk. His robes were scorched and he was dripping wet.
He turned to the four snickering Gryffindors that had started it all, "I hate you."
Sirius just smirked, "It's entirely mutual Snape."
"I WILL HAVE MY REVENGE FOR YOUR EVIL DEEDS SEVERUS SNAPE! MARK MY WORDS YOU SHALL PAY!"
Severus ignored the indignant shouts of the Sorting Hat and left the hall, hopefully Madam Pomfry could do something for his hair.
*snicker* I love torturing Snape, it's so much fun. Anywho, I hope you all enjoyed my explanation for the Sorting Hat's revenge. Review please, they are what I live on.