I 'sped read' your description and missed a couple of important points. What you described makes no sense to me... If the ball was not caught - then a force out at 2nd base, but the batter would not be out. If the ball was caught, batter out and no force out on 2nd base... I agree - mystery to me.
Yes! That's it exactly! It should not have been an unassisted double play, and yet it was! The shortstop catches the ball (batter out) and then steps on second, somehow forcing out the base runner who had been at first! There was no one running from second; these were the two first batters in the Pirate line-up!
I'm beginning to sense an International Baseball Conspiracy, aimed at me personally! It's the game's revenge on me for not being a fan as a kid.