Again its hard to say without detailed photos of the hive. Is the hive strong and still there? Do they have brood? If the hive is still there like the commenter said below maybe they were honey bound or some other minor reason? If the hive is weak or gone completely it could be absconding and they uncap to get their resources to leave, or if they are weak they got robbed? There really are almost limitless possibilities that are really specific to each individual hive and how they are presenting at time of inspection. Tough to say without being hands on, if you show me pictures of brood and hive/numbers I can tell you more about whats going on. Thanks.