Why is my download speed unusually slow?

I normally download games on Steam at about 800 kb/s varying slightly to 700 kb/s up to 1 mb/s. Now I am downloading Borderlands and it is only at a max of 500 kb/s, varying wildly between there and ~100 kb/s, sometimes not even downloading at all. Why? Btw, this is my first question so if I missed something please don't hesitate to tell me.