http://www.wikihow.com/Know-the-Difference-Between-Rise-and-Raise
I too am not sure about the exact reason, but I think..
Raise is a transitive verb - it requires a object to act on
Rise is a intransitive verb - doesnot require an object to act on..
An example could be
She started raising her voice..
here subject is she, object is voice, therefore RAISING is used.
Her voice started rising..
here the subject is the voice itself, therefore RISING is used