When I read the article, if the Earth's core is really spinning ahead of the planet, three possibilities came to mind as to why it is so:
1. the core is denser and heavier, so gravitational centripetal force is greater;
2. drag from the outside in, starting with atmospheric drag, slows the outer portion of the planet;
3. Earth's core sports a mini-micro-black-hole, so more gravity concentrated there, causing slightly accelerated spin.
The first two may be cause, but they are not strong arguments. The last may be possible, but outside current scientific knowledge.