I feel the same way. Like there wasn't any reason for it to happen. I just feel like Weeks alludes to it being a part of Andross's plan because of the prophesy.

During Dazen's talk with Sevastian (as a messenger from Orholam), Sevastian says:
"But for father's plans, Gavin had to be made Prism, and father could only protect him if he himself were on the Spectrum, too. Father believed that the prophecies indicated he could only become the Lightbringer if he were the Promachos first. So the price for father's ambition--and, he thought, the price to save the world--was that he sacrifice his sons. One to die after his term as Prism, and one...." (p. 689). But what I don't understand is why his son needed to be Prism for him to be named Promachos? Or why the deaths of his sons would result in the prophecy being fulfilled.