Air India — Reserved seat changed without prior notice

When i did check-in for ai951 on 19th june 2017, seat selected was 5c and also received check-in confirmation. Upon dropping baggage at airport, boarding pass was issued with 7d. When requested the crew why the change, crew mentioned it was system auto change and they cannot help. I had to board since i have no time. I preferred to have 5c and airline staff denied to assist and was consistently mentioning it was system auto generation and couldn't help. Upon further enquiry it was discovered that the seat was changed to 7d since another passenger opted to pay for 5c which was already reserved for me. Why would paid seats appear for check-in in the first place??