What is the cause of these spicy pillows?

And overcharging too, like if you plug it in 24/7, it's bad

Like, in general?

One of the biggest factors is heat. Lithium batteries don't like high temps.

Also, they really hate sitting at 100% SoC, though I'm not sure if that contributes to the swelling. 80% SoC is commonly cited as a healthier place for lithium batteries to sit when "fully charged," and, for that reason, some laptops (and other devices) allow you to limit the maximum SoC to 80% if your laptop sits plugged in most of the time.