What action can potentially cause an extended boom to topple backwards?

Swinging from the downhill side to the uphill side is particularly hazardous because it places a high load on the crane when the boom is extended. This motion can lead to a shift in the center of gravity, increasing the risk of the crane toppling backwards. When the boom is extended and the swing occurs on uneven ground, the gravitational forces interacting with the load's position can destabilize the crane, especially if combined with an unstable surface or insufficient counterweight.

The other options present different risks but do not directly relate to toppling the crane backwards in the same manner. Moving too quickly can lead to loss of control but does not inherently destabilize the crane in the same way as the swing from a downhill position to an uphill one. Operating in a strong wind can affect the crane's stability, particularly affecting its ability to hold any raised load, but does not specifically address the toppling risk associated with extended booms. Lowering the boom too fast can create a sudden shift in load dynamics, but again, it does not directly correlate with the specific mechanics involved in swinging from a downhill to an uphill side.