Why is it that when a group follows tradition without thinking about the consequences, bad things will happen?

First of all, I have to disagree with the global premise of your question.  When people follow a tradition without thinking of the consequences, bad things do not necessarily happen.  They can and they do, certainly, from time to time, but whether or not this is the case is dependent upon the purpose and nature of the tradition and its potential consequences.   

A literary example that comes to mind is a story in which this is the situation, "The Lottery," by Shirley Jackson.  The ritual is to hold a lottery annually and stone to death the "winner."  It is not clear what the purpose of the tradition is, although it seems to have its roots in some pagan harvest sacrificial ritual.   The people in the story mindlessly adhere to the tradition, disregarding its consequence of death for one person in the village.  So, this is a fictional case of "something bad" happening.

There are real-life examples, too, of course.  Fraternity initiations are one example that comes to me, as is the "tradition" of having 21 shots of alcohol as one turns 21 and is legally permitted to consume alcoholic beverages.  These are rituals that are thoughtlessly carried out with no concern for the consequences, which are physical harm and even death.  There are no doubt other equally idiotic and tragic examples.

However, many people perform rituals that have no bad consequences at all, whether they think about them or not and no matter what their motives are in participating in them. This is true of many religious rituals, for example, taking communion, becoming bar or bat mitzvah, or bringing gifts for the gods in a Hindu temple.  The marriage ceremony is a ritual, too, although this could just as easily be a counter-example, since some people seem to marry thoughtlessly, with less than happy consequence.