This happens when a wither/wither skeleton hits you and damages you with the wither effect. Apparently this also damages the armor while the wither effect is still happening.

How to reproduce:
1. Get some armor and equip it.
2. Get hit by a wither/wither skeleton.
3. You will see that your armor durability is going down despite the effect is the only thing that is damaging your health.

I used a resource pack to determine the durability numbers in the video below:

https://mcpedl.com/durability-viewer-resource-pack/

The unbreaking enchantment can increase the chance for your armor pieces to negate armor durability damage since it apparently counts as damage to armor, however it isn't guaranteed to work since the enchantment is by chance of when the armor can possibly be damaged.