What Causes a Death March?

The cover of Edward Yourdon's book Death March

As we have already discussed, a well managed project has no reason to enter crunch mode for more than a few weeks. Alternatively, any project requiring more is probably flawed from its inception. Edward Yourdon calls these projects Death Marches, which he defines as a project whose 'project parameters' exceed the norm by at least 50 percent. There is a significant amount of ignorance that explains why managers of death march projects think crunch mode is an appropriate response. Beyond that, given that these projects are taxing and have a low probability of success, why are they so frequent?