Typical shrinkage cracks, nothing noteworthy, or anything to do with foundation issues. The cure is simple. You need to hand nail a single 8d finish nail into the face of the molding, about every 24" or so. This nail is installed at a 45* angle and will bite into the top plate of the wall. It should be installed about 1/3rd of the way up from the bottom of the piece. I would pre-drill the molding, and set the nails with the proper nail set. This will eliminate the open joint. The next step is a light application of a paintable caulk, and it will look great. Have fun.