So after the dogs have finished getting their groove on they suddenly realize that neither of them can go their merry ways like a pair of drunk clubbers. They are stuck, they try to pull away from each other but the dogs don't know what is happening why they become stuck and usually just stands there until they pull and realize they are now free.
So lets look at the science of why dogs gets stuck together after mating.
When the dog begins the process of mating, his penis is not erect and is able to penetrate the female because it has an erectile bone called baculm which allows the penis to enter the female once exposed. After the male penetrates the female dog, he holds on much tighter and thrusts deeply. The reason for this deep thrust is to actually get the balbus gland far enough inside the female to be able to trap it. After the female is properly penetrated, the dog begins to rapidly erect and the penis and the balbus gland become swollen.
Male canines are one of the few animals that have a locking bulbus glandis or "bulb", a spherical area of erectile tissue at the base of the penis. During copulation, and only after the male's penis is fully inside the female's vagina, the bulbus glandis becomes engorged with blood, allowing swelling that is described as the knot. When the female's vagina subsequently contracts, the penis becomes locked inside the female. This is known as "tying" or "knotting". While characteristic of mating in most canids, the copulatory tie has been reported to be absent or very brief (less than one minute) in the African wild dog, possibly due to the abundance of large predators in its environment.
So now you know why dogs become locked after intercourse. Happy Dog Breeding!