Trading of these securities in secondary markets is rare. Thus, prices for these securities are not very transparent. Most of these highly structured securities are valued on a "marked-to-model" basis, meaning that statistical models are used to provide values. But the models typically provide only estimates of values, and these estimates can vary widely if there are changes in the underlying assumptions.
Indeed, many of the models assume that the assets backing these securities can be readily traded in a liquid secondary market – an assumption that is clearly not always valid. So it becomes extremely difficult to put a firm value on a particular security at any given time. So, we can now see that many factors made credit markets vulnerable to the recent dislocations.