Kaal Sarp Yog is dosha in kundali is a result of one's past karma. It may also happen if the deceased ancestors are displeased or It may occur if a person has harmed a living being/snake either in the present or the previous life as per Hindu mythology and Astrology.
As per astrology when all the planets (Graha) comes between Rahu and Ketu, then signs of Kaal Sarp Yog as regular health problems/Career issues/family issues/delay in work/loss of mental peace is seen in person.
About the Location:
The Puja is performed on Ghats of Ujjain called as Ramghat which is famous in Ujjain for Kaal Sarp Yoga Dosh Puja.