Through an odd relationship, I have a connection that works at a Discount Tire Store.My father picked up a carriage bolt like that in one of his tires. I too, wondered how the heck that happened.
Usually the front tire sets up the debris to go into the rear tire.
More often than not it is the right side.
And have seen bolts, screws, knives, screw drivers, chisels, other tools really anything that is laying in the street that is substantial can lead to a puncture.