Things to check for:

Signs of gophers--mounds of disturbed soil with a 2" concavity or, rarely, a hole in the side.
Recent digging, as for sprinkler repair, etc.
Recent change in the surrounding landscape, such as grade changes, new pavement, change from grass to gravel, etc.
Heavy fertilizing just before you left, including accidental fertilizer spills.
Any unexplained plant deaths in the neighborhood, which could be a sign of Texas Root Rot in the area. Rare in Nevada, but not unknown. Pines are resistant, but not immune.
Very high winds while you were gone, and cracks in the soil around the trunk--may mean damage to the major roots.
If it's less than 20 feet from the property line,it may have been something your neighbor did (hopefully inadvertently!)
Any other change in the last few months to a year!

Hope this helps! : ]