To my eye it looks like there may have been more water vapor/moisture in the air between the camera and the stars in the second shot than in the first. If the moon was anywhere above the horizon, or there were any ground sources of light pollution in the area, that light would be reflected and disbursed by any thin clouds overhead.

I've been out shooting at night and had very high, very fast moving clouds that aren't even visible to the naked eye show up in photos. They're easier to see in wide angle views when the entire frame is not affected, but the blurrier stars in the second image look a lot like what happens to those stars behind a very thin cloud.

When viewed full screen in a dark environment, the thin clouds can be seen in this view of Orion and surrounding constellations taken March 12, 2013. Those near the horizon are easier to see due to ground based light pollution. But if one looks carefully, one can see a few tendrils of clouds over the area between Orion's belt and Taurus.