I have looked at 10 papadum recipes; none used baking soda, and they are dried in the oven until crisp, very little to no moisture. Is there something about the flour that makes them blister?

It may also be the baking soda used in the papadum dough - often this is one of the ingredients.

I think the puffing has something to do with the water in the dough turning to steam, and it therefore creates a balloon effect.