This might be because each were sold with the old firmware (better noise cancellation), and eventually got updated to the new firmware (better battery).
I believe what is happening is that earwax is building up in the grille on the top inside of the airpod which houses a microphone. As the sound from this microphone gets more distorted, the noise cancellation gradually stops working. You can also see this because after you've had the airpods a couple of months you will never get the ear tip fit test to pass.
Every time this has gotten to the crackling stage, Apple has replaced these without charge, even though I didn't have Applecare. They have some sort of machine in the back of the store that they use to test them, and if your set fails the test, they give you a new pair (sometimes just one).
I recently had my pre-Oct 2020 pros replaced with new manufactured stock under this support program. Within less than a month the replacements developed the same sound issues, supported by Apple's diagnosis and another free replacement.
I don't think I use them in an unusual way - I don't even wear them during exercise. There is just something inherently flawed in the design that causes the sound quality to subtly degrade by a significant amount.