This could almost be anything with the information we are given.
Could have been slightly damaged when you bought them
Could have been damaged when the tires themselves were mounted. Some people aren't that careful
You could have slightly brushed them against something, since they're on the same side
As with most of the damaged tire questions that appear on the site the answer is replace the tire. The side wall is the thinnest part of the tire. The extent of the damage inside the tire is an unknown. While the tire is off ask them to inspect the wheel and try to remove the sharp edges of the damage.