Pitra Dosha is formed if anyone in the family dies unnatural death or it happens many times. It can also happen if family members do not offer water and food to their manes or do not remember their manes in the spiritual activities.
Pitra Dosha occurs if no one in the family is interested to perform spiritual tasks or if any family member kills a cow or destroys a foetus in the womb. In these situations, Sun and Rahu form conjunction in the ninth house in the birth chart of that person who is afflicted by Pitra Dosha.
The best places for pitra dosh puja are ujjain, nasik etc.