The four sports that have events before the ceremonies:
Curling: Never used to be this way, but with the addition of the mixed doubles event (which obviously uses the same venue) it now starts earlier. Looks like they could have moved it up to start the day of the ceremonies as apposed to the day before the ceremonies though as the Women's medal round will lag one day behind the Men's (they may have decided to keep them like that way to have more games at prime times.
Moguls: I don't know why this starts early. I see no reason why it should. However it did start early in 2014 as well. One possible explanation is the new big air event in snowboarding. I took the "virtual tour" of the park on the Olympic website, and it shows the venues for all the freestyle skiing events, and all of the snowboarding events except big air. Perhaps the mogul venue will be converted to the big air venue?
Sk Jumping: Again I don't know why this starts early. All the Ski Jumping will be finished on the 19th.
Figure Skating: The venue is being used for short track as well. Although there are a couple days where there are both going on, and a couple days where neither is going on. It certainly looks like they could fit it all in without forcing anyone to compete too close together. It was early last time too, and that was when the team event was added.
Also of note Snowboarding was early last time.
