This is a serious sign of dryness. This happens when the woods of your guitar become so dry that you lose structural integrity which causes the neck to actually drop down into the sound hole. Sometimes if you catch these guitars in time and begin humidifying them raising humidity back to 60% the drop will stop and you can continue playing these guitars with no real issues. If you do not humidify it will only get worse to where the guitar becomes unplayable and you will soon have major cracks inside the body and around the neck. Repairs like this should only be attempted by a qualified luthier.