The causes can be from what you mentined, as well as the materials used, or 'bumping/vibration', shrinkage, and so on. It's hard to determine what's causing them because, pretty much, all these 'possible causes' could be the reason why.
What's important is that you 'watch' how they grow. If, after a while, they stop, then you can repair them (if you want.) If they continue to get large to the point where they're, like, 1/4" wide, you may want to have the home inspected by someone that specializes in Structural Engineering, or the like. In the meantime, as you know, w/o knowing where the cracks are 'headed', there isn't much you can do. Of course, if you're concerned enough now, you can hire the Engineer now. That's for you to decide.