It's an interesting question. I'm sure there's a good reason, which most of us can only guess at.
One reason we might have played out of the shotgun in game one was Kill's stated desire to help out both Bolin and the O line by going with a lot of very quick throws. Might save a couple seconds.
Will be interesting to see if we continue to play out of the shotgun in this week.
Might be worth emailing in for the upcoming Chris Ash show.