Pimco sees a storm brewing in the $15 trillion U.S. commercial real-estate market that it thinks will cause values to fall up to 5 percent in the next year. Other investors have more dire estimates for a coming sell-off. Their collective pessimism is underpinned by the fact that property loans are coming due, new regulations will most likely prompt building sales and less money will be coming from overseas investors because of slowing growth in China, the Middle East and Europe. And then there's the basic fact that office-building prices have risen so much that they just seem due for a fall.