I think it has something to do with needing more oxygen, just like doing push-ups will help and so will running around or even just stopping your breathing for ~30 seconds. Your body will eventually go into a higher alert state.
It just helps for a short bit though, when you relax your nasal passage does too and blocks all again.
It would by a sympathetic response from catecholamines -- that's how it constricts; however, I didn't know that it had been studied and was actually effective in real life until today.