Why Do Dogs Knot. The normal process of mating in dog couples has different stages, of which last stage is called “tying” or “knotting”. This is called the knot tightening thus preventing the male from withdrawing.
During this phase, the male's penis swells and gets what is referred to as a “knot”, resulting in the male and female dog remaining “locked” or “tied” together, at the male is unable to withdraw. It has one primary role: This “tying” or “knotting” results from swelling of bulbus glandis (also called a knot), which is an erectile tissue structure located on the ventral side of the dog’s penis.