As the fellowship makes their way north through the Vale of Isengard they are suddenly assaulted by Orcs. Why are there orcs in this land?

The Fellowship finds its way to the Wizard's Tower at Isengard. There, they ask for counsel from Curnir the White. Curunir, strangely, counsels them not to go to Imladris, but go rather to the Lonely Mountain and find a piece of the sword in the Dwarven Kingdom of Erebor.