Because along with being a faux Death Eater, Voldemort's right hand man, Order of Phoenix Member, Dumbledore's right hand man, Potions Master, The Half-Blood Prince, Slytherin Head of House, Master Occlumens and a highly skilled Dark Arts Wizard, Snape was also a glorified babysitter.
Somethings to note: That's Snape's wand, and Hermione is waving her right hand like she would during his classes. (She does not in fact have three hands)
I can totally see why Snape was such an ass, having to deal with brats all the time, and dealing with not one master but two. Snape seriously could have used a vacation and should have gotten laid at some point, poor Sevvy