Why do some say my fried chicken tastes like peanut butter?
I like to eat some Asian food for the sole reason it's made with peanut oil. So it's taste like peanut butter but have different texture.
In the case of BK I would say that it's more of a metaphor because people from USA love their peanut butter and it's taste. 

Anyway. If you make your chicken without peanut oil or spices that resemble in taste peanut butter maybe it's the chicken itself? 
I've bought a few times chickens that was feed with fish flour. And guess what - they tasted like fish (not the best thing in the world). Chickens feed with corn have yellowish color and have a cornflakes taste.
So maybe chickens that have been feed with something peanuty?