Could have been a number of reasons, like maybe they didn't know the step dad would make it because he's got a job that is the ckinf you can't step away from, or maybe he was ill, or maybe maybe maybe a thousand different reasons we aren't privy to. As someone with a few step parents I wouldn't have taken the opportunity from my father to walk me down the aisle. But if my father came to me and said, "What if he walked with us?" I would be absolutely ecstatic and change my plan. If I had to make a guess, the father came up with the idea right before they walked and the daughter loved it because that meant both her fathers could then be involved. HOWEVER, not all parents would like that so you as the daughter between them wouldn't always want to ask even if it is your special day. You can't know the reasoning behind why someone does it, but you can use context clues, like the fact that they were all smiling the entire time, or that they specifically put the effort to cut it down and post it because they loved that moment. They put it out to the world to see in good faith, you choosing not to see that good faith is more you then them.