"Force of habit" means something that has the tendency for something to be done frequently. Lord of the Rings is a series of fantasy novels written by J.R.R. Tolkien. "The Hobbit" is a fantasy novel that proceeded the Lord of the Rings series which was also written by J.R.R. Tolkien. The joke is centered around the word "hobbit" since it sounds like "habit" and its use in the common phrase "force of habit", turned into "force of hobbit". 
I keep reading ' The Lord of the Rings ' over and over . I guess it ' s just force of hobbit .