One of the most common reasons is that the septic tank itself is damaged or compromised which causes the waste material inside to leak out into the ground. There are definite signs you will notice if this is happening, but it may take months before the odor and waste product reaches the surface.
Another reason is age. As with most manufactured items, septic tanks are designed to work for a pre-set period before it becomes less efficient and eventually stops working. The older the septic tank, the more likely it is to breakdown.
If a new septic tank is needed, the installation process will be a large, but manageable one. A new septic tank will not only solve all the issues that the old septic tank was no longer addressing but add greater capacity so that it can take care of those living in the home.