you have to remember that the earth never knows where the turn is - it thinks it's moving in a straight line. well, relative to the curvature in space, of course. i know i shouldn't anthropomorphize celestial bodies, especially not on christmas, but the point is that the rotation isn't graceful, or planned out like you would imagine if you were running a race track - the earth is passed out at the wheel. it's nothing like the perfect circle you see in these newtonian diagrams, it's an elongated oval.

so, it's more like that all of the momentum of the earth is pushing directly forward, as though the earth is falling away from the sun (even as it is being propelled forward by it), until it gets to the point where it nearly escapes, and then gets thrown back the other way, like a boomerang.

a boomerang. imagine a boomerang returning back whence it came. that's the earth at the solstice...

so, it's not hard to imagine how these fractures in the crust may end up dealing with stress from that, especially near the equator.