Damage to pan bottom
Baking soda is a mild base (the opposite of being acidic). In the right concentration &amp; environment, it can be a corrosive. 

What happened is that in addition to taking the burned bits off your pan, it also took the shiny finish coat off the outside of the pan. What you're seeing is the unpolished aluminium, with a coating of aluminum oxide. 

Since only the exterior of the pan is affected, you've not damaged the functionality of the pan, just the looks.