There is a condition called leaking air sac (wind puff, subcutaneous emphysema) which can cause air to collect under the skin after an injury near one of the air sacs, but it also can occur with an infection.
http://www.peteducation.com/article.cfm?c=15+1829&aid=2721
https://www.backyardchickens.com/t/171121/deflating-subcutaneous-emphysema
http://www.ijasvm.com/ijasvmadmin/upload/IJASVM_5113cbc51d31b.pdf
http://www.pjbs.org/ijps/fin1339.pdf
http://www.thepoultrysite.com/publications/6/diseases-of-poultry/223/subcutaneous-emphysema/
Air sacs in chickens
