The other is that the effects of the Rashtrakuta invasion, under Krishna III and his brother-in-law Ganga Butuga, and the defeat of the Chola army at Takkolam resulting in the death of heir-apparent Rajaditya Chola (the first in line to the throne - "aanai mael thunjiya devar") must have brought large-scale disorder in the kingdom.
[3] The second theory has more merit since the sons of Parantaka I (specifically Gandaraditya and Arinjaya) must have also fought along with their brother, Rajaditya in that epic battle and must have been variously injured and died rapidly.
Thus, Parantaka I was forced to get his grandson Sundara Chola (the son of Arinjaya and probably the oldest surviving prince) to be the heir-apparent.
As noted earlier, the eldest son of Parantaka I, prince Rajaditya lost his life in the Battle of Takkolam (c. 949 CE).
It is not clear if this is because he was uninterested in war or that he was assimilating his position south of the Paalar River and cutting his losses to keep Eelam (which was fast slipping out of Chola control) and to keep a resurgent Pandya Kingdom at bay.
It is possible that Gandaradhitya was without issue for a long time and in attempt to secure the continuation of the Vijayalaya dynasty, Gandharaditya made his brother heir apparent.
It has been widely accepted by researchers of Tamil literature and Saiva religious scholars that Gandaraditya was the author of a Thiruvisaippa on Siva at the Temple of Chidambaram.
[13] In this there is a distinct statement that Parantaka I conquered the Pandya country and Eelam (Sri Lanka) and covered the temple of Nataraja with gold.