Why is the patent declared invalid after the patent application is successful?

After the patent application is successful, someone suddenly asks to declare its patent invalid. Because there is no proper response, the patent will be successful and invalid. So, who can apply for patent invalidation? What does it mean that the patent application is invalid? Patent invalidation means that after the patent right is granted, it is found that it does not meet the conditions for granting the patent right in the Patent Law and its detailed rules for implementation, and it is re-confirmed by the Patent Reexamination Board and declared invalid. An invalid patent right is regarded as nonexistent from the beginning. Who can apply for patent invalidation? After the patent right has been granted by the patent administration department of the State Council and announced, if any unit or individual thinks that the patent right does not conform to the Patent Law of People's Republic of China (PRC) and its detailed rules for implementation, it may request the the State Council Patent Reexamination Board to declare the announced patent invalid from the date of announcement. Therefore, anyone can apply for patent invalidation, provided that the patent does not comply with relevant regulations. The invalidation of the patent right shall be made by the Patent Reexamination Board and registered and announced by the patent administration department of the State Council.