I have been making the same chocolate-chip cookies for a while. Last week, after baking a batch and removing from the oven, they expanded and became flat.
Usually they stay in the same shape when removed from the oven, (thick and chewy) this time is the only time my cookies really expanded and became flat.
I bake them from the freezer.
What could have gone wrong? too much or too little baking soda? I usually put 1 tsp of baking soda per 3 1/2 cups of regular white flour.