Possible it's changes in weather. If they don't show a lot of symptoms, then it may be a good idea to just monitor them and see how it goes.
New birds on the property, even if not added to this flock could potentially be a source of illness as well unless you have been practicing biosecurity...
If there's no other symptoms and all are active/eating/drinking then I'd just monitor them.
Sneezing can be from a number of things - from weather changes to dust to adding new birds.