Has your dog ever had anaphylaxis, where he/she couldn't breathe? It most likely happens due to an allergy. 

My daughter was walking her dog in the city when the dog keeled over and struggled to breathe. She was fortunate to have a good samaritan help her take the dog to a vet and that saved her life. The vet couldn't see any bites or stings, so they didn't know what happened, but her gums were blue which was a clear sign of an allergic reaction to something. We keep Benadryl on hand for her in case it ever happens again.