Sometimes it can be just caused by dust in the hay, or an allergy.
If there is white mucous from the nose and matted fur on the inside of the front legs where the rabbit has been wiping its’ nose, it is usually caused by a disease called Pasteurellosis.
It is a serious infection, sometimes called “Snuffles”.
A rabbit can sneeze a distance of 6 ft. spreading this air-born infection.
Most rabbits harbor the pasteurella bacteria, without any ill effect.
When health is compromised by stresses, such as heat, cold, fear, giving birth, loss of a companion, or change of owner and environment, pasteurella can take hold.
When it takes hold, it can cause abscesses, pneumonia and death.
You will need to see your vet for appropriate antibiotic treatment immediately when your rabbit starts sneezing, (if there is also white nasal discharge.)
Even with treatment, Pasteurella will stay in their system and any future stress may trigger another attack of infection.
Note I have had a rabbit that makes coughing/sneezing sounds to get attention, so not every sneeze is a sign of Pasteurellosis
Note Rabbits with flat faces, such as Mini Lops and Netherland dwarfs often make a snuffling sound when they breath . This is not the sickness called "snuffles"