My 5 year old Black Lab mix started doing this when he was about a year old. It
scared me to death. I talked to my Vet about it and he said it was "an
elongated soft pallet" that caused it. He said that surgery could be performed
to remove it but healing time would be slow and said it would be a "bloody
mess" until it did. He said it was not life threatening or serious and would
gradually go away as he aged - which it seems to have done.

It rarely happens any more but still scares the life out of me when it does. My
Vet told me that when it happens, just gently cup your palm over the nostrils
to force the dog to breath thru his mouth and it works right away every single
time.