Some people become aggressive when they are depressed, others simply withdrawn. Try and find the root of the depression. Problems at home can easily spill over at school. It's amazing how much a lack of positive attention, problems between parents, or other stresses can impact a child's relationships with his friends. A frustrated child will take it out on his friends until he doesn't have any friends left.
Kids do it because they get away with it.
Enact bold consequences religiously every time they backtalk, are rude, refuse to clean their room, have a tantrum, etc. Make sure your rules are reasonable first. Don't lecture, nag, discuss or argue. Just enact substantial consequences quietly and ruthlessly. And never give in.