We all know those beautiful commercials with aerial photos high in the mountains.  Last week with my Mavic 2 Zoom at about 3,000 meters altitude I noticed that one of my batteries was slightly swollen.  Flying with it was too risky for me.  
Back at sea level, the battery appears less swollen.  When I check the battery condition through the app everything seems OK.  

Hence my question.  Why would the battery begin to swell.  Because of the high altitude?  Worn out?  The battery is just under 3 years old and has always been well maintained.

Hello there TrippleDelta. I am sorry for the trouble this has caused and for sharing these information with us. Battery swelling / bloating 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. In addition, 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. Thank you.

I think you are not the only one.
It is posted here frequently (unfortunately)

Sadly this is too common,   and it always worries me when flying M2P that one will swell and pop out.

How many charges do you have on the battery?

I think DJI has started to make better batteries today. I have read a lot about swollen batteries here and most of them has been manufactured around 2018 and 2019. So your battery being almost 3 years old would fall in this category. Today with batteries manufactured later, they seam to be good.

Battery being a bit old, there may have been some gas in the battery that expanded on that high altitude and then got lesser swelling at lower altitude. Its like blowing up a baloon at sea level and then taking it up like in an air plane and it becomes much bigger.
There is a place where you can check when the battery was manufactured, but I can unfortunately not find this now. Perhaps some one else can help with this.


Edit, Just found the link to where you can check the manufacturing date of your batteries. Just put in the serial number here:  http://tools.retroroms.info/