Sudden death of German Shepherd pup

Question
My GSD pup was approx. 12 weeks old. I had taken him to the vet for his vaccinations, C5 which were administered.
The vet enquired about desexing, we had considered desexing at about 5 - 6 months of age, he stated that it could be done earlier before bad habits developed.
He examined the pup and found that only one testicle had dropped, the other was "in his stomach"he stated that if I had reservarions about desexing that because of this fact it would be necessary at some stage.
I agreed to the operation and left the pup.
Upon arrival at home I received a phone call that the pup had died.
The vet explained that he had administered a tranquilizer  .5 salaprem and .2 atrophen that immediately the pup went into cardiac arrest, he was placed on a resusitator for 15 minutes but was unable to be revived.
Prior to this the pup had been very healthy and well.
The vet explained that 1 in 10-20000 GSD can have a cardiac arrest when administered a tranquilizer, I am devestated by the loss.
Can anyone explain what happened and why
Thanks

Answer
I am very sorry to hear that.  There are some things that very seldom happen, but when they do, it is somebody's puppy.  I really have no idea why it happened.  There is an ask the veterinarian category here.  Perhaps try it.  I have never looked at it and have no idea how good the people there are.