I am quite sure that air prejudices the quality of the coffee. So why some coffee pods are under pressure? Shouldn't they be in vacuum?
Inert gases are known to be chemically stable; they don't react with materials around. Based on that fact Ernesto Illy, the son of the founder of the famous coffee roasting company, invented the method of filling a can of roasted coffee beans with pressurized inert gases. Many years they held the patent for this method and still pressurized roasted coffee cans are the fingerprint for that company. This method is known to protect the roasted beans for a longer period; however it's not magical. So, the beans will stale eventually.
Regarding quality issues, the chances are the gas in the pod isn't air. Perishable products are frequently described as being 'packaged in a protective atmosphere'.
As to why, I'm not sure what the need would be. It could just be a difference in atmospheric pressure between the factory and your location which means the pods bulge.