The descent of prakriti essence via her beautiful creative power known as Maha Kundalini Sakti is there for all those who have eyes to see. The world of phenomena has been created and her work is done and she is now still within the blissful remnants of herself in the vessel at the base of the spine. Who or what was the cause of her descent ?
First verse of Pratyabhijna Hridayam states:
chiti swatantrA vishwasiddhi hetU: (pratyabhijna Hridayam 1.1)
The cause of manifestation of Universe is freedom (swAtantrya) of Universal Consciousness (chiti).
Thus the freedom of consciousness ie, swAtantrya of chiti/Shakti/Mahakundalini is the cause of the manifestation of all this. Thus the cause is just her freedom power (swAtantrya). Regarding how freedom power causes all this to emerge, the power of freedom changes into willpower(Icchya Shakti) which again changes into the power of knowledge (Jnana Shakti) and power of action (Kriya Shakti). Jnana Shakti transforms into subject and Kriya Shakti transforms into the object. So, the cause is just absolute freedom of Universal consciousness (Chiti).
To exercise the absolute power of freedom.
Abhinavagupta says that if creative power (swAtantrya) is not exercised than Almighty would be just like an earthen vessel.
If Almighty God had remained forever in one form, He would have to give up His consciousness and creative power, thus becoming an insentient article like an earthen water vessel (Tantraloka, III. 100- 101).