My boxer does that on occasions. She actually had a period of time when she was doing it all the time and I took her to the vet and we figured out she was allergic to christmas tree flocking! As soon as we took the tree out she was fine. On occasions, she still will do it, but if it constant, it's probably an allergy.
because they have a square muzzle, boxers can take to snorting. It would still be advisable to have a vet check and see if there is a sinus problem or he has inhaled something into his sinuses.