Dogs get tied together after mating due to the male dog's bulbus gland on its penis swelling inside the bitch's vagina. This tie-up can last from anywhere from 2 to 30 minutes and no attempt should be made to separate them.
It is important to try to calm both dogs down when they are stuck together. The male will usually lift one leg over the bitch and stand there, bottom to bottom. The bitch can start to panic and if it starts to try and shake the male dog loose, this can cause penile damage and bruising. It is especially dangerous if the bitch is a lot larger than the male dog.