Yes.
The first is to describe why something HAD TO happen. The second is to describe why something DID happen.Just "the reason" is what it is, and it can be used in three different ways. You used two of...
Reason is the ability of comprehending, inferring, or thinking, and the use of logic. It is otherwise known as the intellect.
The correct expression is "there is a plethora of reasons". Is is used in connection with plethora - plethora is a single object.
a famine