my lab would only hunt about that time until i conditioned him more this year.
but he is dead as door nail when he gets home.
he never had breathing problem as you said but foams at mouth on way to hunt.

it looks like he has rabies.

i only saw what you have 1 time but it was not with a hunting dog.
i saw it with dog at home after running a little ..
i think it was heart problems but dog was 10 years old..

to me, it acts like heart problem but usually thats in older dog..
i saw older beagles do this, they would stand there, tongue out,honking trying to breath,overheated..