Dog Euthenasia

First of all, I'm so sorry you had to witness that.  That had to have been really hard to see a beloved pet in that condition.  Most of the time, the pet just lets their muscles relax and then they're gone.  So I don't know why your dog did that.  Did you ask your Vet?   In 2005, when I had to put my beloved cat of 15 years down, she had screamed and jerked for a few seconds, and my vet told me that it was prob from the injection being cold.  But she settled down right away and then allowed it to take affect.

Reason?  Incompetent veterinarian.  Properly administered, euthanasia should not be that unpleasant.

Take them to court.