How come Te Ka intercepted Maui in the middle of the ocean?

This was addressed in The Art of Moana. In short, Te Ka can travel a reasonable distance from the central island under the ground, emerging from beneath the water.

This is sufficient for her to eventually form a ring of islands some distance from the central island.

As you can see, Maui is escaping from the main island when he's accosted at almost exactly the same point that Moana is blocked from entering the ring of islands protecting the central mound.