The reasons are varied so just keep observing to get a hint of the cause. My cat sneezed when pepper I was using must have puffed out towards him. Did you just shampoo your carpet? Things like that are possible. Are it's eyes sparkly and clear?
It's normal for cats to sneeze. Breeds with flat faces; Persians, Himalayans, Exotics sneeze with an over abundance sometimes because they are plagued with breathing problems. It's helpful to have a humidifier in your home for these breeds because it helps with their breathing.