Puppy suddenly drooling excessively?

Did you have treats with you for his training session? If so, that's probably is it. If you think that Dobes don't/can't drool, you are wrong. Any dog can drool when it is anticipating food. Also, a dog can drool when stressed, such as a dog that hates to go in the car. I've had Dobes for nearly 30 years now, and the *second hand* Dobe I have right now does drool with the anticipation of being fed. Heck, the last show Dobe that I co-owned drooled when it went to the Vet, but never in the ring. One other thing is that dogs often drool before they are going to puke. Honestly, I don't think you are looking at anything serious here unless the dog is acting abnormal.