Alien influence.

It could explain the magic's straight path through the planet, as well as the strong ripple effect. Let's not take the term "alien" as a sci-fi term; rather, let's consider it by definition... an influence that is foreign.

In this case, the alien could be anything from the expected beings from another planet (perhaps using this planet as a grounds for experimentation) to a series of gods or goddesses who have bestowed "great power" in the form of magicks upon the lands. The latter of the two obviously seems more fitting given the direction you're wishing to take the roleplay, but the source is always something that can be debated. Whether or not the source is ever even discovered by the inhabitants of the planet is something that doesn't need to be known right from the start (at least not in terms of the roleplay itself, though it would be nice to know what direction to push this in an out-of-context form).

Hell, the inhabitants don't EVER need to know where the power comes from, if you think about it. Sure, the curious ones could seek answers, but they don't necessarily have to find them if you don't feel like going that route.

What matters is that an otherworldly source produced this stream of magic flowing through the center of the planet, in from one point and out through an opposite point, be it another species or paranormal phenomenon or gods and goddesses.