those desires have not achieved and completed yet than in such a situation their souls don’t get the Moksha or salvation and always wander around us to complete these things. Our ancestors also don’t get moksha or salvation when they wrong deed and sin in their life.
Pitra Dosh Nivaran Mantra
Pitra dosh arises because our forefathers and ancestors have not yet got the salvation or moksha from this world. This happens because when they died at an early age, their death was unnatural and they had a strong feeling to achieve some desires before death.