Here's why:

* Transform boundaries are where two tectonic plates slide past each other horizontally. This creates a lot of friction and stress, leading to earthquakes.

* The Loma Prieta earthquake occurred on the San Andreas Fault, a major transform fault zone in California. This fault is the boundary between the Pacific Plate and the North American Plate.

* While the San Andreas Fault is primarily a transform boundary, it can also have some elements of a strike-slip fault, which contributes to the complexity of the earthquakes that occur there.

Therefore, the Loma Prieta earthquake was a result of the movement along a transform plate boundary.