I do alot of camping and always metal detect around my campsite.
I've found numerous clad coins in the fire pit and around the edges that are bubbled. It most likely occurs when the difference of the two clad materials are heated and begin to seperate from each other.
It's a separation of the metals, but the fact is fire is not always involved.
I have some that have various bubbles - some one side only, some both - some have so little wear that they are still quite shiny, and with NO fire damage.