To understand sneezing, we must first understand bogeys. Read about them here.
The factor that is important is the level of bryophyta microsinus present in the nose.
Under normal conditions this microscopic moss does not cause any reaction in the body and is completely harmless. However in certain cases the growth can get out of hand and excessive amounts of the bryophyta can cause irritation and, ultimately, blockages in the nasal passages. This can be caused by travelling to new places which may have higher levels of the spores in the air. It can also be triggered by high levels of protein in the diet, since it is the amino proteins in mucus that the bryophyta feed on, and more food equals more growth.
The obvious result of clogging is that you are left unable to sneeze out of your nose. The body still has the reflex to sneeze though, and all that power has to go somewhere. Urban legends that a sneeze can reach 100mph are in fact false, but they can reach up to 40mph which is still as fast as a velociraptor trying to escape your face. The only alternative for the body is to allow the sneeze to be expelled through the mouth.