While the other strands lend themselves well to collections of lessons and activities, algebraic thinking really is a way of thinking about mathematics as a whole. In order to teach it effectively, teachers of mathematics must have a grasp of the underlying theory themselves, hence the inclusion of scholarly articles.
Algebraic thinking involves the construction and representation of patterns and regularities, deliberate generalization, and most important, active exploration and conjecture. (from James Kaput, NCTM, 2003)