they're dying.

But why only this one? The other 10 in 3 other areas - some very very close by are doing great?

Yep, I only give them 4 months to live!

I think all of my peppers have some yellow leaves towards the bottom of the plants. It's normal in the heat of the summer. I just pinch them off or leave them be.

Probably some sort of disease, not much you can do. I give it only 3 months to live, maybe a little shorter with an early hard freeze.

If it is still growing okay and producing, let it grow, it may keep producing. Pull off the yellow leaves. If it continues to yellow further up, and it worries you, just dispose of it,.

Bacterial leaf spot?

FWIW, just like animals & humans, some plants of the same variety may be more or less susceptible to diseases and pests.