When kids feel you haven't given them enough attention, they may fight to get more. Although ignoring works for some parents - it requires nerves of steel. Almost any kid can outlast you. If not, go ahead and use the book time out. At least your nerves will survive.
Kids fight when they can get away with it.
In some households one child is clearly the favorite or is perceived as the favorite by the children. In a situation like this, the favored child may instigate a fight simply because he can. Children can be corrupted by power as easily as adults. Be certain the real problem doesn't lie with perceived favoritism on your part. Kids fight when one child feels slighted by the other.