There is no one reason why states collapse. It can happen for a number of reasons. One way to look at this is to say that states collapse because they are too weak or because they become too strong. Sometimes, paradoxically, these can occur together.
Sometimes, states fail because they are too weak. For example, a state can be too weak to repel attacks from its neighbors. Because of this, it collapses and is conquered. Alternatively, a state might not be strong enough to keep the peace among its own people. Conflict might erupt between them and the state collapses.
However, sometimes a state becomes too strong and that, oddly enough, makes it too weak. We can argue that something like this happened with the Roman Empire. It overextended itself when it became too strong. By becoming too strong, it ended up being too weak because it overreached. The same can happen to smaller states. A state might expand to the point where it has a number of ethnic groups within it. At that point, it has becomes too weak to hold together the groups that it incorporated because of its strengths.
Thus, both excessive strength and excessive weakness can cause collapse. However, there are many other things (for example, environmental degradation) that can cause a state to collapse.