Before looking at how Mathuranthaka took the throne, it is important to consider how Aditya was recognized as the heir apparent by Sundara chola. As PS portrays, since mathurantaka was too small when his father (Gandaraditya) and step-father (Arinjaya) died - sundara chola came to throne. But once mathuranthaka came to age, Sundara chola - instead of recognizing him as the next king - makes his own son as the heir apparent. We do not know the reasons behind this - and are not in a position to accept what kalki says - that Paranthaka/Gandaraditya wished Sundara chola and his heirs to continue to claim the chola throne. We do not have any evidence to this. Once good reason could be that Aditya was a great warrior for his age and was - as against what kalki portrays - a pious and intelligent son as well. We will take a look at Aditya in detail later - but for now, let us assume that Aditya was able to surpass Mathurantaka by his own valour..which seems to be the most probable conclusion against the available evidence. No doubt, there were a group of people in the kingdom as well as around Mathuranthaka - who were revolting against this and perhaps Mathuranthaka himself was not happy with this ! (as vividly portrayed in PS) As aditya became more and more famous - installing himself deeper and deeper into the throne - Mathuranthaka may have visualized a may day for him and his generations to come. Moreover, he might have felt that this was wrong - morally and ethically. Sundara should have tried to convice him - saying that a man like Aditya alone can save the country against rising enemy power all around the empire...
It is absolutely possible that our man was not convinced about all this and - in the company of some "dangerous fellows" - decided to play his dice...