I was on the 6/15 cruise where this happened. I was out on the veradah enjoying the sunshine and watching the outlying islands go by, when all the sudden the sun was on the other side of the ship. Didn't think much of it, just figured the ship was changing direction. Then DH came out and said that the captain made an announcement, something to the effect of people being left behind and
DCL being such nice guys that they were going back for them. I thought that was quite odd, as I was under the impression that there was no going back.
I stayed out on the veradah and watched as we returned to port. We got just outside of the harbor area and had to idle a bit as the Carnival ship parked that was also in St. Thomas that day pulled out. Then the Magic pulled in to the harbor with a short gangway (Deck 1, I assume) open. As we got closer, we could see it was a man, woman, and small (my guess is 2yo or less) child we were going back for. I don't recall the man videoing the ship's return, but he did drop down on his knees at one point in thankfulness.
The ship actually just pulled up alongside the dock and lowered the gangway to parallel with the water. The family was in the bed of a small pickup truck, and they sort of jumped from the pickup's tailgate to the gangway (no more than a foot apart, if that). As soon as they were on board, the ship headed back out. By then it was just past 6:30, and I'm pretty sure we had originally left just after 4:30, so the whole adventure put us about 2 hours off schedule.
Waiting in line to get Capt. Henry signature the next day, I heard some stuff from a guy who had attended the officer's Q&A, and some that was probably more along the lines of rumor. What I believe to be true was that the reason the ship went back was that the family had left a child in the Oceaneer's Lab (9yo?), and that Disney cannot take the responsibility for a minor child, so they had to go back for the parents. What may or may not have been true was that although the Magic crew was aware of the no-shows, they didn't make the connection to the child being left on board. They became aware when the child made a fuss about her family not having returned. Also, I heard that the family was out on their own, and got stuck in some kind of traffic situation trying to return, making them late.
I never did hear anything regarding why we went all the way back to port instead of having them come out to the ship, at least part of the way. Nor did I hear anything of what that return trip cost DCL.
The next night (sea day) at the show, cruise director Jim did say that it had put us 2 hours behind schedule, but that the Captain had made up about an hour. We weren't able to go ashore on
Castaway Cay until somewhere around 10:30, so I guess that other hour never got made up. I heard a few people grumble about their time there being cut short.