Curious: why do dogs gets stuck when mating

 

It's called the "Copulatory Tie"

I was so shocked to see them 'stuck' together and even more shocked to discover it was something natural and part of their breeding process. 
Read more after the cut

Apparently, to a great deal of dog breeders, this is something not only common but welcomed.

As usual, I was too darn curious.  I had to research more and this is what I found out:

Unlike the human penis, at the time of insertion, a dog's penis is not erect, and is only able to penetrate the female because it includes a narrow bone called the "baculum" or penile bone, a feature of most placental mammals. When the male achieves penetration, he will usually hold the female tighter and thrust deeper. It is during this time that the male's penis expands and it is important that the bulbus gland is sufficiently far enough inside for the female to be able to trap it. So with canine copulation, the males must first penetrate the female, after which swelling of the penis to erection occurs, which usually happens rapidly.

engorged bulbus glandis

Male canines are the only animals that have 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. 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.

When the penis is locked into the vagina by the bulbus glandis (when the stud is "tied"), the urgency subsides and the male will usually lift a leg and swing it over the female's back while turning around. The two stand with their hind ends touching and the penis locked inside the vagina while ejaculation occurs, decreasing leakage of semen from the vagina.

After some time, typically between 5 and 20 minutes... even up to an hour, the bulbus glandis disengorges, allowing the mates to separate. Virgin dogs can become quite distressed at finding themselves unable to separate during their first copulation, and may try to pull away or run. Dog breeders often suggest it is appropriate for handlers to attempt to calm the mating dogs if they show anxiety once this stage is reached.

And this is what copulatory tying or genital locking is all about!  Got it?  Good, 'cause I wouldn't want to have to explain all that again!