Why does taxiing produce large lateral loads on the 747's engine pylons?
The slip-skid ball indicates the ratio of "sideways" apparent acceleration to "upwards" apparent acceleration, in the aircraft's own reference frame.  In normal constant-airspeed wings-level flight (i.e. not looping or pulling out of a dive), or while taxiing, the "upwards" apparent acceleration is 1 G, so the sideways deflection of the slip-skid ball is a direct indication of the total lateral force acting on the aircraft and the total lateral acceleration experienced by the aircraft.

Turning at a high rate while taxiing will displace the slip-skid ball far to one side.  During high-speed taxi, and the portion of the takeoff and landing where the wheels are on the ground, steering corrections will typically involve low turn rates but may still cause a large displacement of the slip-skid ball due to the high speed involved.  In either case, the large displacement of the skip-skid ball indicates that a strong lateral force is being transmitted to the aircraft, including to the engines through the engine pylons. The fundamental origin of this lateral force is the sideload generated by the tires.

But we can't assume that in actual flight, a similar displacement of the slip-skid ball indicates that a similar sideload is being exerted on the engine pylons.

In actual flight, during a sideslip, the tires are no longer involved.  Instead, the sideways airflow interacts with the various surfaces of the aircraft to generate the aerodynamic sideforce that displaces the slip-slid ball.

Therefore, in flight, the lateral inertial loading created by the mass of the engine whenever the slip-skid ball is displaced, is at least partly offset by the aerodynamic sideforce generated by the engine nacelle.

Consider an aircraft banked to the left, with the slip-skid ball displaced to the left, showing that the airflow is blowing from left to right across the aircraft.  (A "yaw string" would blow toward the right.)  Which is generating more aerodynamic sideforce per unit mass-- the engine nacelles (including the engines contained within), or the rest of the aircraft?  

If the former, the nacelles will be forced toward the right relative to the rest of the aircraft.  If the latter, the nacelles will be forced toward the left relative to the rest of the aircraft.  If the nacelles are generating exactly the same aerodynamic sideforce per unit mass as the rest of the aircraft, then the nacelles will not be forced either to the left or the right relative to the rest of the aircraft, so the pylons will not experience any lateral force at all.

Regardless of which of these three scenarios actually apply, we can be sure that unless the total G-load is quite high, for a given displacement of the slip-slid ball the lateral load on the engine pylon is higher if the aircraft is taxiing than in actual flight.