Recently after I fly, I noticed that my DJI Mavic Air batteries are expanding.  Enough so that it will feel puffy and sometimes pop the case open a little as shown in this pic.  
What gives?  How do I fix it?  

Hello there sbarton. I am sorry for the trouble this has caused and thank you for reaching out. Battery swelling is basically caused by to much heat, vast changes in temperature and/or moisture getting into the battery itself. The lifespan and condition of the battery will depend on the totality of how it was used and stored. Hoping for your kind understanding that every electronic product and batteries has a certain warranty period and lifespan. For safety purposes please do not use bloated or bulging batteries when flying or operating your drone so that the drone's performance will not be affected or compromised . Using bloated or bulging batteries might lead to damaging of the aircraft, or worse, damage to property. Again, I am sorry and thank you for understanding.

Dont fly with these batteries,  when they start to fail like this people loose their drones.

That battery is no longer safe to fly with.  Unfortunately, time to buy a new battery.  

What is the average lifespan of these batteries (recharge/flight count)?  

sbarton Posted at 5-20 06:02
What is the average lifespan of these batteries (recharge/flight count)?

Depends on how well you take care of them. I have had mine for 1.5 years and have had absolutely no swelling and they still work fine. Just remember to try and land between 20 - 30% and dont leave them fully charged for more than 24 hours. Hope this helps