An informational cascade occurs when imperfectly informed people pay attention to the signals given by the views of earlier speakers or actors; those imperfectly informed people join the cascade, thus amplifying the very signals by which they were influenced. If Adam thinks that global warming is a serious problem, Barbara may go along, and Charles may be reluctant to reject the shared view of Adam and Barbara; once Adam, Barbara, and Charles say, and think, that global warming is a serious problem, and once Danielle goes along too, a cascade is starting.
A reputational cascade occurs when people silence themselves because they don't want to be criticized, punished, or ostracized. Those who silence themselves, or seem to go along with the crowd, do not weaken and may even strengthen the reputational pressure on others. If Adam thinks that global warming is a serious problem, Barbara may go along for fear of incurring Adam's disapproval, and Charles may be reluctant to take on both Adam and Barbara; once Danielle goes along too, a reputational cascade is starting.
We are witnessing a Miers cascade, in which many people are giving negative or lukewarm signals largely because of (a) the information contained in the previous signals of others or (b) the reputational pressure to give negative or lukewarm signals. I don't mean to make any evaluation of Ms. Miers' nomination, or even to predict the outcome. But it is clear that we are in the midst of a textbook example of a cascade.