Why did my oatmeal and raisin cookies end up tasting soapy?
Soap is effectively a base (eg baking soda) reacted with a fat (eg oil). Also, heating baking soda can cause it to convert into sodium carbonate, which is a much stronger base.
I would suspect you're on the mark with suspecting the baking soda. The recipe calls for only a quarter of a teaspoon, so it would be very easy too add too much.